linux-kernel - Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader optimistic spinning

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Date:   Tue, 4 Jun 2019 11:10:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader
 optimistic spinning

On Mon, May 20, 2019 at 04:59:14PM -0400, Waiman Long wrote:
> Reader optimistic spinning is helpful when the reader critical section
> is short and there aren't that many readers around. It makes readers
> relatively more preferred than writers. When a writer times out spinning
> on a reader-owned lock and set the nospinnable bits, there are two main
> reasons for that.
> 
>  1) The reader critical section is long, perhaps the task sleeps after
>     acquiring the read lock.
>  2) There are just too many readers contending the lock causing it to
>     take a while to service all of them.
> 
> In the former case, long reader critical section will impede the progress
> of writers which is usually more important for system performance.
> In the later case, reader optimistic spinning tends to make the reader
> groups that contain readers that acquire the lock together smaller
> leading to more of them. That may hurt performance in some cases. In
> other words, the setting of nonspinnable bits indicates that reader
> optimistic spinning may not be helpful for those workloads that cause it.
> 
> Therefore, any writers that have observed the setting of the writer
> nonspinnable bit for a given rwsem after they fail to acquire the lock
> via optimistic spinning will set the reader nonspinnable bit once they
> acquire the write lock. Similarly, readers that observe the setting
> of reader nonspinnable bit at slowpath entry will also set the reader
> nonspinnable bit when they acquire the read lock via the wakeup path.

So both cases set the _reader_ nonspinnable bit?