Re: [Qemu-block] [PATCH v2 11/13] block/mirror: remove block_job_sleep_ns calls

[Max Reitz]

On 2018-01-19 21:58, John Snow wrote:
> We're attempting to slacken the mirror loop in three different places,
> but we can combine these three attempts. Combine the early loop call to
> block_job_pause_point with the two late-loop calls to block_job_sleep_ns.
> 
> When delay_ns is 0 and it has not been SLICE_TIME since the last yield,
> block_job_relax is merely a call to block_job_pause_point, so this should
> be equivalent with the exception that if we have managed to not yield at
> all in the last SLICE_TIME ns, we will now do so.
> 
> I am not sure that condition was possible,
> so this loop should be equivalent.

Well, to me it even sounds like a positive change if it was a change.
We want the job to yield after SLICE_TIME ns, after all, and I don't
think it matters where that happens, exactly.

> 
> Signed-off-by: John Snow <address@hidden>
> ---
>  block/mirror.c     | 22 +++++++++++-----------
>  block/trace-events |  2 +-
>  2 files changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index a0e0044de2..192e03694f 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -761,7 +761,7 @@ static void coroutine_fn mirror_run(void *opaque)
>      assert(!s->dbi);
>      s->dbi = bdrv_dirty_iter_new(s->dirty_bitmap);
>      for (;;) {
> -        uint64_t delay_ns = 0;
> +        static uint64_t delay_ns = 0;

Errr.  Are you sure about that?

Now every mirror job in the qeny process will share this single
variable.  Was that your intention?

>          int64_t cnt, delta;
>          bool should_complete;
>  
> @@ -770,9 +770,16 @@ static void coroutine_fn mirror_run(void *opaque)
>              goto immediate_exit;
>          }
>  
> -        block_job_pause_point(&s->common);
> -
>          cnt = bdrv_get_dirty_count(s->dirty_bitmap);
> +
> +        trace_mirror_before_relax(s, cnt, s->synced, delay_ns);
> +        if (block_job_relax(&s->common, delay_ns)) {

See the reply to <a href="HEAD^^">that patch</a>.

> +            if (!s->synced) {
> +                goto immediate_exit;
> +            }
> +        }
> +        delay_ns = 0;
> +
>          /* s->common.offset contains the number of bytes already processed so
>           * far, cnt is the number of dirty bytes remaining and
>           * s->bytes_in_flight is the number of bytes currently being
> @@ -849,15 +856,8 @@ static void coroutine_fn mirror_run(void *opaque)
>          }
>  
>          ret = 0;
> -        trace_mirror_before_sleep(s, cnt, s->synced, delay_ns);
> -        if (!s->synced) {
> -            block_job_sleep_ns(&s->common, delay_ns);
> -            if (block_job_is_cancelled(&s->common)) {
> -                break;
> -            }
> -        } else if (!should_complete) {
> +        if (s->synced && !should_complete) {
>              delay_ns = (s->in_flight == 0 && cnt == 0 ? SLICE_TIME : 0);
> -            block_job_sleep_ns(&s->common, delay_ns);
>          }
>      }

Basic idea looks good to me (apart from the static delay_ns), but, well,
block-job-cancel on a busy job still breaks.

Max

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