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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <cb524fd4-0c74-4fc0-6ddd-96f37a83fe64@redhat.com>
Date:   Tue, 4 Jun 2019 13:28:35 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 6/4/19 5:10 AM, Peter Zijlstra wrote:
> 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?

Yes.

-Longman