Re: [Qemu-devel] [PATCH 07/12] migration: hold the lock only if it is really needed

[Xiao Guangrong]

On 06/29/2018 07:22 PM, Dr. David Alan Gilbert wrote:
> * Xiao Guangrong (address@hidden) wrote:
> > On 06/19/2018 03:36 PM, Peter Xu wrote:
> > > On Mon, Jun 04, 2018 at 05:55:15PM +0800, address@hidden wrote:
> > > > From: Xiao Guangrong <address@hidden>
> > > >
> > > > Try to hold src_page_req_mutex only if the queue is not
> > > > empty
> > >
> > > Pure question: how much this patch would help?  Basically if you are
> > > running compression tests then I think it means you are with precopy
> > > (since postcopy cannot work with compression yet), then here the lock
> > > has no contention at all.
> >
> > Yes, you are right, however we can observe it is in the top functions
> > (after revert this patch):
>
> Can you show the matching trace with the patch in?

Sure, there is:

+   8.38%  kqemu  [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
+   8.03%  kqemu  qemu-system-x86_64       [.] ram_bytes_total
+   6.62%  kqemu  qemu-system-x86_64       [.] qemu_event_set
+   6.02%  kqemu  qemu-system-x86_64       [.] qemu_put_qemu_file
+   5.81%  kqemu  qemu-system-x86_64       [.] __ring_put
+   5.04%  kqemu  qemu-system-x86_64       [.] compress_thread_data_done
+   4.48%  kqemu  qemu-system-x86_64       [.] ring_is_full
+   4.44%  kqemu  qemu-system-x86_64       [.] ring_mp_get
+   3.39%  kqemu  qemu-system-x86_64       [.] __ring_is_full
+   2.61%  kqemu  qemu-system-x86_64       [.] add_to_iovec
+   2.48%  kqemu  qemu-system-x86_64       [.] threads_submit_request_prepare
+   2.08%  kqemu  libc-2.12.so             [.] memcpy
+   2.07%  kqemu  qemu-system-x86_64       [.] ring_len
+   1.91%  kqemu  [kernel.kallsyms]        [k] __lock_acquire
+   1.60%  kqemu  qemu-system-x86_64       [.] buffer_zero_sse2
+   1.16%  kqemu  qemu-system-x86_64       [.] ram_find_and_save_block
+   1.14%  kqemu  qemu-system-x86_64       [.] ram_save_target_page
+   1.12%  kqemu  qemu-system-x86_64       [.] compress_page_with_multi_thread
+   1.09%  kqemu  qemu-system-x86_64       [.] ram_save_host_page
+   1.07%  kqemu  qemu-system-x86_64       [.] test_and_clear_bit
+   1.07%  kqemu  qemu-system-x86_64       [.] qemu_put_buffer
+   1.03%  kqemu  qemu-system-x86_64       [.] qemu_put_byte
+   0.80%  kqemu  qemu-system-x86_64       [.] threads_submit_request_commit
+   0.74%  kqemu  qemu-system-x86_64       [.] migration_bitmap_clear_dirty
+   0.70%  kqemu  qemu-system-x86_64       [.] control_save_page
+   0.69%  kqemu  qemu-system-x86_64       [.] test_bit
+   0.69%  kqemu  qemu-system-x86_64       [.] ram_save_iterate
+   0.63%  kqemu  qemu-system-x86_64       [.] migration_bitmap_find_dirty
+   0.63%  kqemu  qemu-system-x86_64       [.] ram_control_save_page
+   0.62%  kqemu  qemu-system-x86_64       [.] rcu_read_lock
+   0.56%  kqemu  qemu-system-x86_64       [.] qemu_file_get_error
+   0.55%  kqemu  [kernel.kallsyms]        [k] lock_acquire
+   0.55%  kqemu  qemu-system-x86_64       [.] find_dirty_block
+   0.54%  kqemu  qemu-system-x86_64       [.] ring_index
+   0.53%  kqemu  qemu-system-x86_64       [.] ring_put
+   0.51%  kqemu  qemu-system-x86_64       [.] unqueue_page
+   0.50%  kqemu  qemu-system-x86_64       [.] migrate_use_compression
+   0.48%  kqemu  qemu-system-x86_64       [.] get_queued_page
+   0.46%  kqemu  qemu-system-x86_64       [.] ring_get
+   0.46%  kqemu  [i40e]                   [k] i40e_clean_tx_irq
+   0.45%  kqemu  [kernel.kallsyms]        [k] lock_release
+   0.44%  kqemu  [kernel.kallsyms]        [k] native_sched_clock
+   0.38%  kqemu  qemu-system-x86_64       [.] migrate_get_current
+   0.38%  kqemu  [kernel.kallsyms]        [k] find_held_lock
+   0.34%  kqemu  [kernel.kallsyms]        [k] __lock_release
+   0.34%  kqemu  qemu-system-x86_64       [.] qemu_ram_pagesize
+   0.29%  kqemu  [kernel.kallsyms]        [k] lock_is_held_type
+   0.27%  kqemu  [kernel.kallsyms]        [k] update_load_avg
+   0.27%  kqemu  qemu-system-x86_64       [.] save_page_use_compression
+   0.24%  kqemu  qemu-system-x86_64       [.] qemu_file_rate_limit
+   0.23%  kqemu  [kernel.kallsyms]        [k] tcp_sendmsg
+   0.23%  kqemu  [kernel.kallsyms]        [k] match_held_lock
+   0.22%  kqemu  [kernel.kallsyms]        [k] do_raw_spin_trylock
+   0.22%  kqemu  [kernel.kallsyms]        [k] cyc2ns_read_begin