linux-kernel - Re: SLUB: purpose of sysfs events on cache creation/removal

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Date:   Sat, 18 Jan 2020 16:15:28 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: SLUB: purpose of sysfs events on cache creation/removal

On Fri, 17 Jan 2020 18:13:31 +0100 Michal Koutný <mkoutny@...e.com> wrote:

> Hello.
> 
> On Thu, Jan 09, 2020 at 11:44:15AM -0800, Andrew Morton <akpm@...ux-foundation.org> wrote:
> > I looked at it - there wasn't really any compelling followup.
> FTR, I noticed udevd consuming non-negligible CPU cycles when doing some
> cgroup stress testing. And even extrapolating to less artificial
> situations, the udev events seem to cause useless tickling of udevd.
> 
> I used the simple script below
> cat measure.sh <<EOD
> sample() {
>         local n=$(echo|awk "END {print int(40/$1)}")
> 
>         for i in $(seq $n) ; do
>                 mkdir /sys/fs/cgroup/memory/grp1 ;
>                 echo 0 >/sys/fs/cgroup/memory/grp1/cgroup.procs ;
>                 /usr/bin/sleep $1 ;
>                 echo 0 >/sys/fs/cgroup/memory/cgroup.procs ;
>                 rmdir /sys/fs/cgroup/memory/grp1 ;
>         done
> }
> 
> for d in 0.004 0.008 0.016 0.032 0.064 0.128 0.256 0.5 1 ; do
>         echo 0 >/sys/fs/cgroup/cpuacct/system.slice/systemd-udevd.service/cpuacct.usage
>         time sample $d 2>&1 | grep real
>         echo -n "udev "
>         cat /sys/fs/cgroup/cpuacct/system.slice/systemd-udevd.service/cpuacct.usage
> done
> EOD
> 
> and I drew the following ballpark conclusion:
> 1.7% CPU time at 1 event/s -> 60 event/s 100% cpu
> 
> (The event is one mkdir/migrate/rmdir sequence. Numbers are from dummy
> test VM, so take with a grain of salt.)

Thanks.  What effect does this patch have upon these numbers?

> 
> > If this change should be pursued then can we please have a formal
> > resend?
> Who's supposed to do that?

Typically the author, but not always.  If someone else is particularly
motivated to get a patch merged up they can take it over.