linux-kernel - Re: [PATCH 0/7] proc/stat: Maintain monotonicity of "intr" and "softirq"

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Message-ID: <87y27zb62e.ffs@tglx>
Date:   Tue, 14 Sep 2021 16:11:21 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexei Lozovsky <me@...mmy.net>,
        Alexey Dobriyan <adobriyan@...il.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/7] proc/stat: Maintain monotonicity of "intr" and
 "softirq"

On Sun, Sep 12 2021 at 21:37, Alexei Lozovsky wrote:
> On Sun, Sep 12, 2021, at 18:30, Alexey Dobriyan wrote:
>> How about making everything "unsigned long" or even "u64" like NIC
>> drivers do?
>
> I see some possible hurdles ahead:
>
> - Not all architectures have atomic operations for 64-bit values

This is not about atomics.

>   All those "unsigned int" counters are incremented with __this_cpu_inc()
>   which tries to use atomics if possible. Though, I'm not quite sure

It does not use atomics. It's a CPU local increment.

>   how this works for read side which does not seem to use atomic reads
>   at all. I guess, just by the virtue of properly aligned 32-bit reads
>   being atomic everywhere? If that's so, I think widening counters to
>   64 bits will come with an asterisk.

The stats are accumulated racy, i.e. the interrupt might be handled and
one of the per cpu counters or irq_desc->tot_count might be incremented
concurrently.

On 32bit systems a 32bit load (as long as the compiler does not emit
load tearing) is always consistent even when there is a concurrent
increment going on. It either gets the old or the new value.

A 64bit read on a 32bit system is always two loads which means that a
concurrent increment will make it possible to observe a half updated
value. And no, you can't play reread tricks here without adding barriers
on weakly ordered architectures.

> - We'll need to update all counters to be 64-bit.
>
>   Like, *everyone*. Every field that gets summed up needs to be 64-bit
>   (or else wrap-arounds will be incorrect). Basically every counter in
>   every irq_cpustat_t will need to become twice as wide. If that's
>   a fine price to pay for accurate, full-width counters...

The storage size should not be a problem.

> So right now I don't see why it shouldn't be doable in theory.

So much for the theory :)

Thanks,

        tglx