linux-kernel - Re: [PATCH v6 0/1] proc: Allow pid_revalidate() during LOOKUP

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Date:   Mon, 4 Oct 2021 19:41:36 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Stephen Brennan <stephen.s.brennan@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Konrad Wilk <konrad.wilk@...cle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v6 0/1] proc: Allow pid_revalidate() during LOOKUP_RCU

On Mon, Oct 04, 2021 at 07:20:14PM +0100, Matthew Wilcox wrote:
> On Mon, Oct 04, 2021 at 10:56:28AM -0700, Stephen Brennan wrote:
> > Problem Description:
> > 
> > When running running ~128 parallel instances of "TZ=/etc/localtime ps
> > -fe >/dev/null" on a 128CPU machine, the %sys utilization reaches 97%,
> > and perf shows the following code path as being responsible for heavy
> > contention on the d_lockref spinlock:
> > 
> >       walk_component()
> >         lookup_fast()
> >           d_revalidate()
> >             pid_revalidate() // returns -ECHILD
> >           unlazy_child()
> >             lockref_get_not_dead(&nd->path.dentry->d_lockref) <-- contention
> > 
> > The reason is that pid_revalidate() is triggering a drop from RCU to ref
> > path walk mode. All concurrent path lookups thus try to grab a reference
> > to the dentry for /proc/, before re-executing pid_revalidate() and then
> > stepping into the /proc/$pid directory. Thus there is huge spinlock
> > contention. This patch allows pid_revalidate() to execute in RCU mode,
> > meaning that the path lookup can successfully enter the /proc/$pid
> > directory while still in RCU mode. Later on, the path lookup may still
> > drop into ref mode, but the contention will be much reduced at this
> > point.
> > 
> > By applying this patch, %sys utilization falls to around 85% under the
> > same workload, and the number of ps processes executed per unit time
> > increases by 3x-4x. Although this particular workload is a bit
> > contrived, we have seen some large collections of eager monitoring
> > scripts which produced similarly high %sys time due to contention in the
> > /proc directory.
> 
> I think it's perhaps also worth noting that this is a performance
> regression relative to ... v5.4?  v4.14?  I forget the details; do you
> have those to hand, Stephen?
> 
> (Yes, this is a stupid workload.  Yes, a customer really does have
> this workload.)

OK, it's not a performance regression.  My apologies; I misremembered
the ticket.  What happens with 4.14 is that the tasks all sleep on
the directory's i_mutex.  When i_mutex became i_rwsem, all the lookups
would now contend on the dentry spinlock.  That turns "lots of processes
sleeping" into "lots of processes spinning", which looks like a regression
if you're asking "Why has my system time increased a lot?"