linux-kernel - Re: [PATCH v4 0/5] kernfs: proposed locking and concurrency improvement

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Date:   Thu, 13 May 2021 23:37:38 +0800
From:   Fox Chen <foxhlchen@...il.com>
To:     Ian Kent <raven@...maw.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>, Al Viro <viro@...iv.linux.org.uk>,
        Eric Sandeen <sandeen@...deen.net>,
        Brice Goglin <brice.goglin@...il.com>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 0/5] kernfs: proposed locking and concurrency improvement

Hi Ian

On Thu, May 13, 2021 at 10:10 PM Ian Kent <raven@...maw.net> wrote:
>
> On Wed, 2021-05-12 at 16:54 +0800, Fox Chen wrote:
> > On Wed, May 12, 2021 at 4:47 PM Fox Chen <foxhlchen@...il.com> wrote:
> > >
> > > Hi,
> > >
> > > I ran it on my benchmark (
> > > https://github.com/foxhlchen/sysfs_benchmark).
> > >
> > > machine: aws c5 (Intel Xeon with 96 logical cores)
> > > kernel: v5.12
> > > benchmark: create 96 threads and bind them to each core then run
> > > open+read+close on a sysfs file simultaneously for 1000 times.
> > > result:
> > > Without the patchset, an open+read+close operation takes 550-570
> > > us,
> > > perf shows significant time(>40%) spending on mutex_lock.
> > > After applying it, it takes 410-440 us for that operation and perf
> > > shows only ~4% time on mutex_lock.
> > >
> > > It's weird, I don't see a huge performance boost compared to v2,
> > > even
> >
> > I meant I don't see a huge performance boost here and it's way worse
> > than v2.
> > IIRC, for v2 fastest one only takes 40us
>
> Thanks Fox,
>
> I'll have a look at those reports but this is puzzling.
>
> Perhaps the added overhead of the check if an update is
> needed is taking more than expected and more than just
> taking the lock and being done with it. Then there's
> the v2 series ... I'll see if I can dig out your reports
> on those too.

Apologies, I was mistaken, it's compared to V3, not V2.  The previous
benchmark report is here.
https://lore.kernel.org/linux-fsdevel/CAC2o3DKNc=sL2n8291Dpiyb0bRHaX=nd33ogvO_LkJqpBj-YmA@mail.gmail.com/

> >
> >
> > > though there is no mutex problem from the perf report.
> > > I've put console outputs and perf reports on the attachment for
> > > your reference.
>
> Yep, thanks.
> Ian
>

thanks,
fox