linux-kernel - Re: [PATCH] psi:fix divide by zero in psi_update

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Message-ID: <20191112154844.GD168812@cmpxchg.org>
Date:   Tue, 12 Nov 2019 10:48:44 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     tim <xiejingfeng@...ux.alibaba.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH] psi:fix divide by zero in psi_update_stats

On Tue, Nov 12, 2019 at 10:41:46AM -0500, Johannes Weiner wrote:
> On Fri, Nov 08, 2019 at 03:33:24PM +0800, tim wrote:
> > In psi_update_stats, it is possible that period has value like
> > 0xXXXXXXXX00000000 where the lower 32 bit is 0, then it calls div_u64 which
> > truncates u64 period to u32, results in zero divisor.
> > Use div64_u64() instead of div_u64()  if the divisor is u64 to avoid
> > truncation to 32-bit on 64-bit platforms.
> > 
> > Signed-off-by: xiejingfeng <xiejingfeng@...ux.alibaba.com>
> 
> This is legit. When we stop the periodic averaging worker due to an
> idle CPU, the period after restart can be much longer than the ~4 sec
> in the lower 32 bits. See the missed_periods logic in update_averages.

Argh, that's not right. Of course I notice right after hitting send.

missed_periods are subtracted out of the difference between now and
the last update, so period should be not much bigger than 2s.

Something else is going on here.