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From: peterz@infradead.org
Subject: Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks
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Date: Thu, 23 Jul 2020 21:58:55 +0200
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To: Waiman Long <longman@redhat.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>, Nicholas Piggin <npiggin@gmail.com>, linuxppc-dev@lists.ozlabs.org, Will Deacon <will@kernel.org>, Boqun Feng <boqun.feng@gmail.com>, Ingo Molnar <mingo@redhat.com>, Anton Blanchard <anton@ozlabs.org>, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm-ppc@vger.kernel.org, linux-arch@vger.kernel.org

On Thu, Jul 23, 2020 at 03:04:13PM -0400, Waiman Long wrote:
> On 7/23/20 2:47 PM, peterz@infradead.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.