Re: [RFC PATCH] mce: prevent concurrent polling of MCE events

[Aristeu Rozanski <aris@ruivo.org>]

On Thu, May 11, 2023 at 04:38:57PM -0700, Tony Luck wrote:
> I just ran a test on a system booted with mce=no_cmci and saw the
> duplicate reporting.  Digging into the code I see that I'm not the
> first to think that a little randomness in timers on different CPUs
> would be a good idea. The MCE code starts/restarts timers with:
> 
> 	mod_timer(t, round_jiffies(when));
> 
> Looking at the round_jiffies() function I sw that it is grabbing
> the current CPU number.
> 
> unsigned long round_jiffies(unsigned long j)
> {
>         return round_jiffies_common(j, raw_smp_processor_id(), false);
> }
> 
> And round_jiffies_common() explains with this comment:
> 
>         /*
>          * We don't want all cpus firing their timers at once hitting the
>          * same lock or cachelines, so we skew each extra cpu with an extra
>          * 3 jiffies. This 3 jiffies came originally from the mm/ code which
>          * already did this.
>          * The skew is done by adding 3*cpunr, then round, then subtract this
>          * extra offset again.
>          */
>         j += cpu * 3;
> 
> 	rem = j % HZ;
> 
> So your original patch https://lore.kernel.org/all/Y8WtE7BNJ0gTrqIS@cathedrallabs.org/
> that just added the spinlock should be fine!

Thanks Tony. You want me to resend it or the original is enough?

-- 
Aristeu