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

[Jens Axboe]

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:

Great!

>  - 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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