linux-kernel - Re: [PATCH v2 1/2] Revert "locking/rwsem: Remove reader optimistic spinning"

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Message-ID: <20230920040908.GA4197@KORCO045595.samsungds.net>
Date:   Wed, 20 Sep 2023 13:09:08 +0900
From:   Bongkyu Kim <bongkyu7.kim@...sung.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
        will@...nel.org, boqun.feng@...il.com,
        linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        bongkyu7.kim@...sung.com
Subject: Re: [PATCH v2 1/2] Revert
 "locking/rwsem: Remove reader optimistic spinning"

On Wed, Sep 06, 2023 at 09:32:45AM -0400, Waiman Long wrote:
> 
> On 9/6/23 07:27, Bongkyu Kim wrote:
> > On Mon, Sep 04, 2023 at 03:56:56PM -0400, Waiman Long wrote:
> > > On 9/4/23 11:10, Peter Zijlstra wrote:
> > > > On Fri, Sep 01, 2023 at 10:07:03AM +0900, Bongkyu Kim wrote:
> > > > > This reverts commit 617f3ef95177840c77f59c2aec1029d27d5547d6.
> > > > > 
> > > > > In mobile environment, reader optimistic spinning is still useful
> > > > > because there're not many readers. In my test result at android device,
> > > > > it improves application startup time about 3.8%
> > > > > App startup time is most important factor for android user expriences.
> > > > > So, re-enable reader optimistic spinning by this commit. And,
> > > > > the later patch will make it optional feature by cmdline.
> > > > I'm not seeing any mention on how this interacts with all the rwsem work
> > > > that has been done since that commit, like the handoff rework.
> > > > 
> > > > Why is a straight revert a sane thing at this point?
> > > I also agree that a revert is not the best way to reintroduce the feature.
> > > It should document the reason why reader optimistic spinning is not the
> > > default as discussed in commit 617f3ef9517 ("locking/rwsem: Remove reader
> > > optimistic spinning") and under what condition should reader optimistic
> > > spinning can be turned back on.
> > > 
> > > Besides, I now think we may not really need 2 separate nonspinnable bits. We
> > > can go with one that is set by writer timing out when spinning on reader.
> > > 
> > > Cheers,
> > > Longman
> > Should I modify like the below?
> > - Title to "locking/rwsem: Reintroduce reader optimistic spinning"
> > - Add more document like Longman's comment
> > - Reconsidering about 2 separate nonspinnable bits to one
> 
> Besides the above, Peter also ask to verify that it won't affect handoff
> handling which requires that an unlocker see the lock will be free and wake
> up the head of the wait queue. Given the fact that the simple heuristic of
> skipping optimistic spinning if the lock is reader owned is kept, that
> shouldn't be a problem, but you still need to document that.
> 
> Cheers,
> Longman
 
I've been reviewing nonspinnable bits for several days, but I can't find the
way for one spinnable bit. In this patch, how about modify only already mentioned
document part? About one spinnable bit, we will discuss later.