Re: [PATCH 05/10] block: remove per-queue plugging

From: Jens Axboe
Date: Tue Apr 12 2011 - 04:36:54 EST

On 2011-04-12 03:12, hch@xxxxxxxxxxxxx wrote:
> On Mon, Apr 11, 2011 at 02:48:45PM +0200, Jens Axboe wrote:
>> Great, once you do that and XFS kills the blk_flush_plug() calls too,
>> then we can remove that export and make it internal only.
> Linus pulled the tree, so they are gone now. Btw, there's still some
> bits in the area that confuse me:


> - what's the point of the queue_sync_plugs? It has a lot of comment
> that seem to pre-data the onstack plugging, but except for that
> it's trivial wrapper around blk_flush_plug, with an argument
> that is not used.

There's really no point to it anymore. It's existance was due to the
older revision that had to track write requests for serializaing around
a barrier. I'll kill it, since we don't do that anymore.

> - is there a good reason for the existance of __blk_flush_plug? You'd
> get one additional instruction in the inlined version of
> blk_flush_plug when opencoding, but avoid the need for chained
> function calls.
> - Why is having a plug in blk_flush_plug marked unlikely? Note that
> unlikely is the static branch prediction hint to mark the case
> extremly unlikely and is even used for hot/cold partitioning. But
> when we call it we usually check beforehand if we actually have
> plugs, so it's actually likely to happen.

The existance and out-of-line is for the scheduler() hook. It should be
an unlikely event to schedule with a plug held, normally the plug should
have been explicitly unplugged before that happens.

> - what is the point of blk_finish_plug? All callers have
> the plug on stack, and there's no good reason for adding the NULL
> check. Note that blk_start_plug doesn't have the NULL check either.

That one can probably go, I need to double check that part since some
things changed.

> - Why does __blk_flush_plug call __blk_finish_plug which might clear
> tsk->plug, just to set it back after the call? When manually inlining
> __blk_finish_plug ino __blk_flush_plug it looks like:
> void __blk_flush_plug(struct task_struct *tsk, struct blk_plug *plug)
> {
> flush_plug_list(plug);
> if (plug == tsk->plug)
> tsk->plug = NULL;
> tsk->plug = plug;
> }
> it would seem much smarted to just call flush_plug_list directly.
> In fact it seems like the tsk->plug is not nessecary at all and
> all remaining __blk_flush_plug callers could be replaced with
> flush_plug_list.

It depends on whether this was an explicit unplug (eg
blk_finish_plug()), or whether it was an implicit event (eg on
schedule()). If we do it on schedule(), then we retain the plug after
the flush. Otherwise we clear it.

> - and of course the remaining issue of why io_schedule needs an
> expliciy blk_flush_plug when schedule() already does one in
> case it actually needs to schedule.

Already answered in other email.

Jens Axboe

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