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From:   Pavel Tatashin <pasha.tatashin@soleen.com>
Date:   Wed, 11 Mar 2020 16:25:51 -0400
Message-ID: <CA+CK2bD5J2ZWHF3UX0b4oH5zYSLCqKfgHZMoPXg-PJAxfeN0oQ@mail.gmail.com>
Subject: Re: [PATCH v3] mm: fix tick timer stall during deferred page init
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     Shile Zhang <shile.zhang@linux.alibaba.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Mar 11, 2020 at 4:17 PM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>
> Hi, Pasha,
>
> On 11.03.2020 20:45, Pavel Tatashin wrote:
> > On Wed, Mar 11, 2020 at 8:39 AM Shile Zhang
> > <shile.zhang@linux.alibaba.com> wrote:
> >>
> >> When 'CONFIG_DEFERRED_STRUCT_PAGE_INIT' is set, 'pgdatinit' kthread will
> >> initialise the deferred pages with local interrupts disabled. It is
> >> introduced by commit 3a2d7fa8a3d5 ("mm: disable interrupts while
> >> initializing deferred pages").
> >>
> >> On machine with NCPUS <= 2, the 'pgdatinit' kthread could be bound to
> >> the boot CPU, which could caused the tick timer long time stall, system
> >> jiffies not be updated in time.
> >>
> >> The dmesg shown that:
> >>
> >>     [    0.197975] node 0 initialised, 32170688 pages in 1ms
> >>
> >> Obviously, 1ms is unreasonable.
> >>
> >> Now, fix it by restore in the pending interrupts for every 32*1204 pages
> >> (128MB) initialized, give the chance to update the systemd jiffies.
> >> The reasonable demsg shown likes:
> >>
> >>     [    1.069306] node 0 initialised, 32203456 pages in 894ms
> >
> > Sorry for joining late to this thread. I wonder if we could use
> > sched_clock() to print this statistics. Or not to print statistics at
> > all?
>
> This won't work for all cases since sched_clock() may fall back to jiffies-based
> implementation, which gives wrong result, when interrupts are disabled.
>
> And a bigger problem is not a statistics, but it's advancing jiffies. Some parallel
> thread may expect jiffies are incrementing, and this will be a surprise for that
> another component.
>
> So, this fix is more about modularity and about not introduction a new corner case.

Makes sense.

Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>

Thank you,
Pasha