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Re: [PATCH v6] 9pfs: use GHashTable for fid table
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From: |
Christian Schoenebeck |
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Subject: |
Re: [PATCH v6] 9pfs: use GHashTable for fid table |
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Date: |
Wed, 05 Oct 2022 11:38:39 +0200 |
On Dienstag, 4. Oktober 2022 14:54:16 CEST Christian Schoenebeck wrote:
> On Dienstag, 4. Oktober 2022 12:41:21 CEST Linus Heckemann wrote:
> > The previous implementation would iterate over the fid table for
> > lookup operations, resulting in an operation with O(n) complexity on
> > the number of open files and poor cache locality -- for every open,
> > stat, read, write, etc operation.
> >
> > This change uses a hashtable for this instead, significantly improving
> > the performance of the 9p filesystem. The runtime of NixOS's simple
> > installer test, which copies ~122k files totalling ~1.8GiB from 9p,
> > decreased by a factor of about 10.
> >
> > Signed-off-by: Linus Heckemann <git@sphalerite.org>
> > Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> > Reviewed-by: Greg Kurz <groug@kaod.org>
> > Message-Id: <20220908112353.289267-1-git@sphalerite.org>
> > [CS: - Retain BUG_ON(f->clunked) in get_fid().
> >
> > - Add TODO comment in clunk_fid(). ]
> >
> > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > ---
>
> In general: LGTM now, but I will definitely go for some longer test runs
> before queuing this patch. Some minor side notes below ...
So I was running a compilation marathon on 9p as root fs this night, first
couple hours went smooth, but then after about 12 hours 9p became unusable
with error:
Too many open files
The question is, is that a new issue introduced by this patch? I.e. does it
break the reclaim fd code? Or is that rather unrelated to this patch, and a
problem we already had?
Linus, could you look at this? It would probably make sense to force getting
into this situation much earlier like:
diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index aebadeaa03..0c104b81e1 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -4330,6 +4330,6 @@ static void __attribute__((__constructor__))
v9fs_set_fd_limit(void)
error_report("Failed to get the resource limit");
exit(1);
}
- open_fd_hw = rlim.rlim_cur - MIN(400, rlim.rlim_cur / 3);
+ open_fd_hw = rlim.rlim_cur - MIN(50, rlim.rlim_cur / 3);
open_fd_rc = rlim.rlim_cur / 2;
}
I can't remember that we had this issue before, so there might still be
something wrong with this GHashTable patch here.
> > Christian Schoenebeck writes:
> > > Not a big deal, but I start thinking whether to keep BUG_ON() here as
> > > well.
> > > That would require using g_hash_table_lookup() here instead of
> > > g_hash_table_contains(). Not that I would insist.
> >
> > Done.
> >
> > > checkpatch.pl
> >
> > Fixed.
> >
> > > OK, I get it that you squashed your previous patch and that you ended up
> > > with this comment instead. But if you read the old comment version here,
> > > you'll notice that it describes the actual problem more accurately:
> > > especially that v9fs_reopen_fid() and put_fid() are the 2 functions that
> > > may cause the coroutine to "yield" here. That's an important information
> > > to be retained in this comment here as it's not obvious.
> >
> > Reworded to mention these functions explicitly.
> >
> > > I would not move this to the 2nd loop. I would still set the
> > >
> > > FID_NON_RECLAIMABLE flag here, for the same reasons that you are bumping
> > > the refcount already in the first loop below.
> >
> > Good point! Fixed.
> >
> > > ... that's not the same behaviour as before, is it? Old behaviour was
> > > to
> > > put>
> > >
> > > the respective (last) fid only on error. And this would now put all
> > > fids.
> >
> > Indeed it isn't the old behaviour, but I believe it's correct: since
> > before we only incremented the reference counter for each one when we
> > started reopening it. Now, we increment _all_ their refcounts, so we
> > need to put all of them back as well.
>
> Yeah, commented in v9fs_mark_fids_unreclaim() changes below ...
>
> > > Wasn't there something like GLIST_FOREACH()? Then you wouldn't need to
> > > add
> > >
> > > that variable.
> >
> > glib does provide g_list_foreach, but that requires a function
> > pointer. I'm not aware of a macro for this. I could change this to use
> > a QLIST (for which we do have a macro) instead of a GList if you
> > insist, but I'd default to leaving this as is.
>
> That's better handled by a future cleanup patch. Right now I find it
> prioritized to get this performance fix merged ASAP, as it provides a
> significant improvement for a lot of people.
>
> I have seen GLIST_FOREACH macros (without function pointer) in other
> projects, but I guess that macro was defined locally by those projects.
>
> > Thanks for your repeated reviews and patience!
> >
> > Linus
> >
> > hw/9pfs/9p.c | 195 +++++++++++++++++++++++++++++----------------------
> > hw/9pfs/9p.h | 2 +-
> > 2 files changed, 113 insertions(+), 84 deletions(-)
> >
> > diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> > index aebadeaa03..729d3181e0 100644
> > --- a/hw/9pfs/9p.c
> > +++ b/hw/9pfs/9p.c
> > @@ -256,7 +256,8 @@ static size_t v9fs_string_size(V9fsString *str)
> >
> > }
> >
> > /*
> >
> > - * returns 0 if fid got re-opened, 1 if not, < 0 on error */
> > + * returns 0 if fid got re-opened, 1 if not, < 0 on error
> > + */
> >
> > static int coroutine_fn v9fs_reopen_fid(V9fsPDU *pdu, V9fsFidState *f)
> > {
> >
> > int err = 1;
> >
> > @@ -282,33 +283,32 @@ static V9fsFidState *coroutine_fn get_fid(V9fsPDU
> > *pdu, int32_t fid) V9fsFidState *f;
> >
> > V9fsState *s = pdu->s;
> >
> > - QSIMPLEQ_FOREACH(f, &s->fid_list, next) {
> > + f = g_hash_table_lookup(s->fids, GINT_TO_POINTER(fid));
> > + if (f) {
> >
> > BUG_ON(f->clunked);
> >
> > - if (f->fid == fid) {
> > - /*
> > - * Update the fid ref upfront so that
> > - * we don't get reclaimed when we yield
> > - * in open later.
> > - */
> > - f->ref++;
> > - /*
> > - * check whether we need to reopen the
> > - * file. We might have closed the fd
> > - * while trying to free up some file
> > - * descriptors.
> > - */
> > - err = v9fs_reopen_fid(pdu, f);
> > - if (err < 0) {
> > - f->ref--;
> > - return NULL;
> > - }
> > - /*
> > - * Mark the fid as referenced so that the LRU
> > - * reclaim won't close the file descriptor
> > - */
> > - f->flags |= FID_REFERENCED;
> > - return f;
> > + /*
> > + * Update the fid ref upfront so that
> > + * we don't get reclaimed when we yield
> > + * in open later.
> > + */
> > + f->ref++;
> > + /*
> > + * check whether we need to reopen the
> > + * file. We might have closed the fd
> > + * while trying to free up some file
> > + * descriptors.
> > + */
> > + err = v9fs_reopen_fid(pdu, f);
> > + if (err < 0) {
> > + f->ref--;
> > + return NULL;
> >
> > }
> >
> > + /*
> > + * Mark the fid as referenced so that the LRU
> > + * reclaim won't close the file descriptor
> > + */
> > + f->flags |= FID_REFERENCED;
> > + return f;
> >
> > }
> > return NULL;
> >
> > }
> >
> > @@ -317,12 +317,11 @@ static V9fsFidState *alloc_fid(V9fsState *s, int32_t
> > fid) {
> >
> > V9fsFidState *f;
> >
> > - QSIMPLEQ_FOREACH(f, &s->fid_list, next) {
> > + f = g_hash_table_lookup(s->fids, GINT_TO_POINTER(fid));
> > + if (f) {
> >
> > /* If fid is already there return NULL */
> > BUG_ON(f->clunked);
> >
> > - if (f->fid == fid) {
> > - return NULL;
> > - }
> > + return NULL;
> >
> > }
> > f = g_new0(V9fsFidState, 1);
> > f->fid = fid;
> >
> > @@ -333,7 +332,7 @@ static V9fsFidState *alloc_fid(V9fsState *s, int32_t
> > fid) * reclaim won't close the file descriptor
> >
> > */
> >
> > f->flags |= FID_REFERENCED;
> >
> > - QSIMPLEQ_INSERT_TAIL(&s->fid_list, f, next);
> > + g_hash_table_insert(s->fids, GINT_TO_POINTER(fid), f);
> >
> > v9fs_readdir_init(s->proto_version, &f->fs.dir);
> > v9fs_readdir_init(s->proto_version, &f->fs_reclaim.dir);
> >
> > @@ -424,12 +423,12 @@ static V9fsFidState *clunk_fid(V9fsState *s, int32_t
> > fid) {
> >
> > V9fsFidState *fidp;
> >
> > - QSIMPLEQ_FOREACH(fidp, &s->fid_list, next) {
> > - if (fidp->fid == fid) {
> > - QSIMPLEQ_REMOVE(&s->fid_list, fidp, V9fsFidState, next);
> > - fidp->clunked = true;
> > - return fidp;
> > - }
> > + /* TODO: Use g_hash_table_steal_extended() instead? */
> > + fidp = g_hash_table_lookup(s->fids, GINT_TO_POINTER(fid));
> > + if (fidp) {
> > + g_hash_table_remove(s->fids, GINT_TO_POINTER(fid));
> > + fidp->clunked = true;
> > + return fidp;
> >
> > }
> > return NULL;
> >
> > }
> >
> > @@ -439,10 +438,15 @@ void coroutine_fn v9fs_reclaim_fd(V9fsPDU *pdu)
> >
> > int reclaim_count = 0;
> > V9fsState *s = pdu->s;
> > V9fsFidState *f;
> >
> > + GHashTableIter iter;
> > + gpointer fid;
> > +
> > + g_hash_table_iter_init(&iter, s->fids);
> > +
> >
> > QSLIST_HEAD(, V9fsFidState) reclaim_list =
> >
> > QSLIST_HEAD_INITIALIZER(reclaim_list);
> >
> > - QSIMPLEQ_FOREACH(f, &s->fid_list, next) {
> > + while (g_hash_table_iter_next(&iter, &fid, (gpointer *) &f)) {
> >
> > /*
> >
> > * Unlink fids cannot be reclaimed. Check
> > * for them and skip them. Also skip fids
> >
> > @@ -514,72 +518,86 @@ void coroutine_fn v9fs_reclaim_fd(V9fsPDU *pdu)
> >
> > }
> >
> > }
> >
> > +/*
> > + * This is used when a path is removed from the directory tree. Any
> > + * fids that still reference it must not be closed from then on, since
> > + * they cannot be reopened.
> > + */
> >
> > static int coroutine_fn v9fs_mark_fids_unreclaim(V9fsPDU *pdu, V9fsPath
> >
> > *path) {
> > - int err;
> > + int err = 0;
>
> Not really necessary as `err` was and is never used unitialized, but OK.
>
> > V9fsState *s = pdu->s;
> >
> > - V9fsFidState *fidp, *fidp_next;
> > + V9fsFidState *fidp;
> > + gpointer fid;
> > + GHashTableIter iter;
> > + /*
> > + * The most common case is probably that we have exactly one
> > + * fid for the given path, so preallocate exactly one.
> > + */
> > + g_autoptr(GArray) to_reopen = g_array_sized_new(FALSE, FALSE,
> > + sizeof(V9fsFidState *), 1);
> > + gint i;
> >
> > - fidp = QSIMPLEQ_FIRST(&s->fid_list);
> > - if (!fidp) {
> > - return 0;
> > - }
> > + g_hash_table_iter_init(&iter, s->fids);
> >
> > /*
> >
> > - * v9fs_reopen_fid() can yield : a reference on the fid must be held
> > - * to ensure its pointer remains valid and we can safely pass it to
> > - * QSIMPLEQ_NEXT(). The corresponding put_fid() can also yield so
> > - * we must keep a reference on the next fid as well. So the logic
> > here
> > - * is to get a reference on a fid and only put it back during the
> > next
> > - * iteration after we could get a reference on the next fid. Start
> > with - * the first one.
> > + * We iterate over the fid table looking for the entries we need
> > + * to reopen, and store them in to_reopen. This is because
> > + * v9fs_reopen_fid() and put_fid() yield. This allows the fid table
> > + * to be modified in the meantime, invalidating our iterator.
> >
> > */
> >
> > - for (fidp->ref++; fidp; fidp = fidp_next) {
> > + while (g_hash_table_iter_next(&iter, &fid, (gpointer *) &fidp)) {
> >
> > if (fidp->path.size == path->size &&
> >
> > !memcmp(fidp->path.data, path->data, path->size)) {
> >
> > - /* Mark the fid non reclaimable. */
> > - fidp->flags |= FID_NON_RECLAIMABLE;
> > -
> > - /* reopen the file/dir if already closed */
> > - err = v9fs_reopen_fid(pdu, fidp);
> > - if (err < 0) {
> > - put_fid(pdu, fidp);
> > - return err;
> > - }
> > - }
> > -
> > - fidp_next = QSIMPLEQ_NEXT(fidp, next);
> > -
> > - if (fidp_next) {
> >
> > /*
> >
> > - * Ensure the next fid survives a potential clunk request
> > during
> > - * put_fid() below and v9fs_reopen_fid() in the next
> > iteration.
> > + * Ensure the fid survives a potential clunk
> > request during
> > + * v9fs_reopen_fid or put_fid.
> >
> > */
> >
> > - fidp_next->ref++;
> > + fidp->ref++;
> > + fidp->flags |= FID_NON_RECLAIMABLE;
> > + g_array_append_val(to_reopen, fidp);
> >
> > }
> >
> > + }
> >
> > - /* We're done with this fid */
> > - put_fid(pdu, fidp);
> > + for (i = 0; i < to_reopen->len; i++) {
> > + fidp = g_array_index(to_reopen, V9fsFidState*, i);
> > + /* reopen the file/dir if already closed */
> > + err = v9fs_reopen_fid(pdu, fidp);
> > + if (err < 0) {
> > + goto out;
>
> Looks like a simple `break;` here and dropping the `out:` label below is
> sufficient. But I can do that on my end.
>
> > + }
> >
> > }
> >
> > - return 0;
> > + out:
> > + for (i = 0; i < to_reopen->len; i++) {
> > + put_fid(pdu, g_array_index(to_reopen, V9fsFidState*, i));
> > + }
> > + return err;
> >
> > }
>
> ... so yeah, I think that simple put_fid() loop is fine. Probably I
> overlooked the 2nd put_fid() call in the original code. So basically the
> old == new behaviour was and is: the sum of refcount increments/decrements
> after returning from this function is exactly neutral/zero for each fid,
> without any exception.
>
> > static void coroutine_fn virtfs_reset(V9fsPDU *pdu)
> > {
> >
> > V9fsState *s = pdu->s;
> > V9fsFidState *fidp;
> >
> > + GList *freeing;
> > + /*
> > + * Get a list of all the values (fid states) in the table, which
> > + * we then...
> > + */
> > + g_autoptr(GList) fids = g_hash_table_get_values(s->fids);
> >
> > - /* Free all fids */
> > - while (!QSIMPLEQ_EMPTY(&s->fid_list)) {
> > - /* Get fid */
> > - fidp = QSIMPLEQ_FIRST(&s->fid_list);
> > - fidp->ref++;
> > + /* ... remove from the table, taking over ownership. */
> > + g_hash_table_steal_all(s->fids);
> >
> > - /* Clunk fid */
> > - QSIMPLEQ_REMOVE(&s->fid_list, fidp, V9fsFidState, next);
> > + /*
> > + * This allows us to release our references to them asynchronously
> > without
> > + * iterating over the hash table and risking iterator
> > invalidation
> > + * through concurrent modifications.
> > + */
> > + for (freeing = fids; freeing; freeing = freeing->next) {
> > + fidp = freeing->data;
> > + fidp->ref++;
> >
> > fidp->clunked = true;
> >
> > -
> >
> > put_fid(pdu, fidp);
> >
> > }
> >
> > }
> >
> > @@ -3205,6 +3223,8 @@ static int coroutine_fn v9fs_complete_rename(V9fsPDU
> > *pdu, V9fsFidState *fidp, V9fsFidState *tfidp;
> >
> > V9fsState *s = pdu->s;
> > V9fsFidState *dirfidp = NULL;
> >
> > + GHashTableIter iter;
> > + gpointer fid;
> >
> > v9fs_path_init(&new_path);
> > if (newdirfid != -1) {
> >
> > @@ -3238,11 +3258,13 @@ static int coroutine_fn
> > v9fs_complete_rename(V9fsPDU *pdu, V9fsFidState *fidp, if (err < 0) {
> >
> > goto out;
> >
> > }
> >
> > +
> >
> > /*
> >
> > * Fixup fid's pointing to the old name to
> > * start pointing to the new name
> > */
> >
> > - QSIMPLEQ_FOREACH(tfidp, &s->fid_list, next) {
> > + g_hash_table_iter_init(&iter, s->fids);
> > + while (g_hash_table_iter_next(&iter, &fid, (gpointer *) &tfidp)) {
> >
> > if (v9fs_path_is_ancestor(&fidp->path, &tfidp->path)) {
> >
> > /* replace the name */
> > v9fs_fix_path(&tfidp->path, &new_path,
> >
> > strlen(fidp->path.data)); @@ -3320,6 +3342,8 @@ static int coroutine_fn
> > v9fs_fix_fid_paths(V9fsPDU *pdu, V9fsPath *olddir, V9fsPath oldpath,
> > newpath;
> >
> > V9fsState *s = pdu->s;
> > int err;
> >
> > + GHashTableIter iter;
> > + gpointer fid;
> >
> > v9fs_path_init(&oldpath);
> > v9fs_path_init(&newpath);
> >
> > @@ -3336,7 +3360,8 @@ static int coroutine_fn v9fs_fix_fid_paths(V9fsPDU
> > *pdu, V9fsPath *olddir, * Fixup fid's pointing to the old name to
> >
> > * start pointing to the new name
> > */
> >
> > - QSIMPLEQ_FOREACH(tfidp, &s->fid_list, next) {
> > + g_hash_table_iter_init(&iter, s->fids);
> > + while (g_hash_table_iter_next(&iter, &fid, (gpointer *) &tfidp)) {
> >
> > if (v9fs_path_is_ancestor(&oldpath, &tfidp->path)) {
> >
> > /* replace the name */
> > v9fs_fix_path(&tfidp->path, &newpath, strlen(oldpath.data));
> >
> > @@ -4226,7 +4251,7 @@ int v9fs_device_realize_common(V9fsState *s, const
> > V9fsTransport *t, s->ctx.fmode = fse->fmode;
> >
> > s->ctx.dmode = fse->dmode;
> >
> > - QSIMPLEQ_INIT(&s->fid_list);
> > + s->fids = g_hash_table_new(NULL, NULL);
> >
> > qemu_co_rwlock_init(&s->rename_lock);
> >
> > if (s->ops->init(&s->ctx, errp) < 0) {
> >
> > @@ -4286,6 +4311,10 @@ void v9fs_device_unrealize_common(V9fsState *s)
> >
> > if (s->ctx.fst) {
> >
> > fsdev_throttle_cleanup(s->ctx.fst);
> >
> > }
> >
> > + if (s->fids) {
> > + g_hash_table_destroy(s->fids);
> > + s->fids = NULL;
> > + }
> >
> > g_free(s->tag);
> > qp_table_destroy(&s->qpd_table);
> > qp_table_destroy(&s->qpp_table);
> >
> > diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
> > index 994f952600..10fd2076c2 100644
> > --- a/hw/9pfs/9p.h
> > +++ b/hw/9pfs/9p.h
> > @@ -339,7 +339,7 @@ typedef struct {
> >
> > struct V9fsState {
> >
> > QLIST_HEAD(, V9fsPDU) free_list;
> > QLIST_HEAD(, V9fsPDU) active_list;
> >
> > - QSIMPLEQ_HEAD(, V9fsFidState) fid_list;
> > + GHashTable *fids;
> >
> > FileOperations *ops;
> > FsContext ctx;
> > char *tag;
> >
> > --
> > 2.36.2