Messages in this thread Patch in this message | | | From | Jeff Moyer <> | Subject | Re: [PATCH] blk: clean up plug | Date | Fri, 17 Apr 2015 16:54:40 -0400 |
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Shaohua Li <shli@fb.com> writes:
> Current code looks like inner plug gets flushed with a > blk_finish_plug(). Actually it's a nop. All requests/callbacks are added > to current->plug, while only outmost plug is assigned to current->plug. > So inner plug always has empty request/callback list, which makes > blk_flush_plug_list() a nop. This tries to make the code more clear. > > Signed-off-by: Shaohua Li <shli@fb.com>
Hi, Shaohua,
I agree that this looks like a clean-up with no behavioral change, and it looks good to me. However,it does make me scratch my head about the numbers I was seeing. Here's the table from that other email thread[1]:
device| vanilla | patch1 | dio-noplug | noflush-nested ------+------------+----------------+---------------+----------------- rssda | 701,684 | 1,168,527 | 1,342,177 | 1,297,612 | 100% | +66% | +91% | +85% vdb0 | 358,264 | 902,913 | 906,850 | 922,327 | 100% | +152% | +153% | +157%
Patch1 refers to the first patch in this series, which fixes the merge logic for single-queue blk-mq devices. Each column after that includes that first patch. In dio-noplug, I removed the blk_plug from the direct-io code path (so there is no nesting at all). This is a control, since it is what I expect the outcome of the noflush-nested column to actually be. Then, the noflush-nested column leaves the blk_plug in place in the dio code, but includes the patch that prevents nested blk_plug's from being flushed. All numbers are the average of 5 runs. With the exception of the vanilla run on rssda (the first run was faster, causing the average to go up), the standard deviation is very small.
For the dio-noplug column, if the inner plug really was a noop, then why would we see any change in performance? Like I said, I agree with your reading of the code and the patch. Color me confused. I'll poke at it more next week. For now, I think your patch is fine.
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Also, Jens or Shaohua or anyone, please review my blk-mq plug fix (patch 1/2 of aforementioned thread). ;)
-Jeff
[1] https://lkml.org/lkml/2015/4/16/366
> --- > block/blk-core.c | 24 ++++++++++++------------ > 1 file changed, 12 insertions(+), 12 deletions(-) > > diff --git a/block/blk-core.c b/block/blk-core.c > index 794c3e7..d3161f3 100644 > --- a/block/blk-core.c > +++ b/block/blk-core.c > @@ -3018,21 +3018,20 @@ void blk_start_plug(struct blk_plug *plug) > { > struct task_struct *tsk = current; > > + /* > + * If this is a nested plug, don't actually assign it. > + */ > + if (tsk->plug) > + return; > + > INIT_LIST_HEAD(&plug->list); > INIT_LIST_HEAD(&plug->mq_list); > INIT_LIST_HEAD(&plug->cb_list); > - > /* > - * If this is a nested plug, don't actually assign it. It will be > - * flushed on its own. > + * Store ordering should not be needed here, since a potential > + * preempt will imply a full memory barrier > */ > - if (!tsk->plug) { > - /* > - * Store ordering should not be needed here, since a potential > - * preempt will imply a full memory barrier > - */ > - tsk->plug = plug; > - } > + tsk->plug = plug; > } > EXPORT_SYMBOL(blk_start_plug); > > @@ -3179,10 +3178,11 @@ void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule) > > void blk_finish_plug(struct blk_plug *plug) > { > + if (plug != current->plug) > + return; > blk_flush_plug_list(plug, false); > > - if (plug == current->plug) > - current->plug = NULL; > + current->plug = NULL; > } > EXPORT_SYMBOL(blk_finish_plug);
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