linux-kernel - Re: [PATCH -next v4] blk-mq: fix tag

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Date:   Wed, 12 Jan 2022 18:29:28 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     John Garry <john.garry@...wei.com>,
        QiuLaibin <qiulaibin@...wei.com>, ming.lei@...hat.com,
        martin.petersen@...cle.com, hare@...e.de,
        johannes.thumshirn@....com, bvanassche@....org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v4] blk-mq: fix tag_get wait task can't be awakened

On Wed, Jan 12, 2022 at 08:37:34AM -0700, Jens Axboe wrote:
> On 1/12/22 7:38 AM, Andy Shevchenko wrote:
> > On Wed, Jan 12, 2022 at 12:51:13PM +0000, John Garry wrote:
> >> On 12/01/2022 12:30, Andy Shevchenko wrote:
> >>>>>> +		if (test_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags) ||
> >>>>>> +		    test_and_set_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags)) {
> >>>>> Whoever wrote this code did too much defensive programming, because the first
> >>>>> conditional doesn't make much sense here. Am I right?
> >>>>>
> >>>> I think because this judgement is in the general IO process, there are also
> >>>> some performance considerations here.
> >>> I didn't buy this. Is there any better argument why you need redundant
> >>> test_bit() call?
> >>
> >> I think that the idea is that test_bit() is fast and test_and_set_bit() is
> >> slow; as such, if we generally expect the bit to be set, then there is no
> >> need to do the slower test_and_set_bit() always.
> > 
> > It doesn't sound thought through solution, the bit can be flipped in
> > between, so what is this all about? Maybe missing proper serialization
> > somewhere else?
> 
> You need to work on your communication a bit - if there's a piece of
> code you don't understand, maybe try being a bit less aggressive about
> it? Otherwise people tend to just ignore you rather than explain it.

Sure. Thanks for below explanations, btw.

> test_bit() is a lot faster than a test_and_set_bit(), and there's no
> need to run the latter if the former returns true. This is a pretty
> common optimization, particularly if the majority of the calls end up
> having the bit set already.

I don't see how it may give optimization here generally speaking.
If we know that bit is _often_ is set, than of course it sounds
like opportunistic win. Otherwise, it may have the opposite effect.

I.o.w. without knowing the statistics of the bit being set/clear it's
hard to say if it's optimization or waste of the (CPU) resources.

> Can the bit be flipped right after? Certainly! Can that happen if just
> test_and_set_bit() is used? Of course! This isn't a critical section
> with a lock, it's a piece of state. And guarding the RMW operation with
> a test first doesn't change that one bit.

-- 
With Best Regards,
Andy Shevchenko