linux-kernel - Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR

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Date:   Thu, 23 Jul 2020 17:58:43 -0400
From:   Waiman Long <longman@...hat.com>
To:     peterz@...radead.org
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        linuxppc-dev@...ts.ozlabs.org, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Anton Blanchard <anton@...abs.org>,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm-ppc@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for
 SPLPAR

On 7/23/20 3:58 PM, peterz@...radead.org wrote:
> On Thu, Jul 23, 2020 at 03:04:13PM -0400, Waiman Long wrote:
>> On 7/23/20 2:47 PM, peterz@...radead.org wrote:
>>> On Thu, Jul 23, 2020 at 02:32:36PM -0400, Waiman Long wrote:
>>>> BTW, do you have any comment on my v2 lock holder cpu info qspinlock patch?
>>>> I will have to update the patch to fix the reported 0-day test problem, but
>>>> I want to collect other feedback before sending out v3.
>>> I want to say I hate it all, it adds instructions to a path we spend an
>>> aweful lot of time optimizing without really getting anything back for
>>> it.
>> It does add some extra instruction that may slow it down slightly, but I
>> don't agree that it gives nothing back. The cpu lock holder information can
>> be useful in analyzing crash dumps and in some debugging situation. I think
>> it can be useful in RHEL for this readon. How about an x86 config option to
>> allow distros to decide if they want to have it enabled? I will make sure
>> that it will have no performance degradation if the option is not enabled.
> Config knobs suck too; they create a maintenance burden (we get to make
> sure all the permutations works/build/etc..) and effectively nobody uses
> them, since world+dog uses what distros pick.
>
> Anyway, instead of adding a second per-cpu variable, can you see how
> horrible something like this is:
>
> unsigned char adds(unsigned char var, unsigned char val)
> {
> 	unsigned short sat = 0xff, tmp = var;
>
> 	asm ("addb	%[val], %b[var];"
> 	     "cmovc	%[sat], %[var];"
> 	     : [var] "+r" (tmp)
> 	     : [val] "ir" (val), [sat] "r" (sat)
> 	     );
>
> 	return tmp;
> }
>
> Another thing to try is, instead of threading that lockval throughout
> the thing, simply:
>
> #define _Q_LOCKED_VAL	this_cpu_read_stable(cpu_sat)
>
> or combined with the above
>
> #define _Q_LOCKED_VAL	adds(this_cpu_read_stable(cpu_number), 2)
>
> and see if the compiler really makes a mess of things.
>
Thanks for the suggestion. I will try that out.

Cheers,
Longman