From: Waiman Long <longman@redhat.com>
To: peterz@infradead.org
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
Subject: Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR
Date: Thu, 23 Jul 2020 17:58:43 -0400 [thread overview]
Message-ID: <4fbe47a5-dbc9-427a-5b21-b31b37bc751a@redhat.com> (raw)
In-Reply-To: <20200723195855.GU119549@hirez.programming.kicks-ass.net>
On 7/23/20 3:58 PM, peterz@infradead.org wrote:
> 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.
>
Thanks for the suggestion. I will try that out.
Cheers,
Longman
next prev parent reply other threads:[~2020-07-23 21:58 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-06 4:35 [PATCH v3 0/6] powerpc: queued spinlocks and rwlocks Nicholas Piggin
2020-07-06 4:35 ` [PATCH v3 1/6] powerpc/powernv: must include hvcall.h to get PAPR defines Nicholas Piggin
2020-07-09 10:05 ` Michael Ellerman
2020-07-06 4:35 ` [PATCH v3 2/6] powerpc/pseries: move some PAPR paravirt functions to their own file Nicholas Piggin
2020-07-09 10:11 ` Michael Ellerman
2020-07-06 4:35 ` [PATCH v3 3/6] powerpc: move spinlock implementation to simple_spinlock Nicholas Piggin
2020-07-09 10:15 ` Michael Ellerman
2020-07-06 4:35 ` [PATCH v3 4/6] powerpc/64s: implement queued spinlocks and rwlocks Nicholas Piggin
2020-07-09 10:20 ` Michael Ellerman
2020-07-09 10:33 ` Peter Zijlstra
2020-07-23 14:37 ` Michal Suchánek
2020-07-06 4:35 ` [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR Nicholas Piggin
2020-07-09 10:53 ` Michael Ellerman
2020-07-09 11:03 ` Peter Zijlstra
2020-07-09 16:06 ` Waiman Long
2020-07-23 14:00 ` Peter Zijlstra
2020-07-23 18:32 ` Waiman Long
2020-07-23 18:47 ` peterz
2020-07-23 19:04 ` Waiman Long
2020-07-23 19:58 ` peterz
2020-07-23 20:30 ` Segher Boessenkool
2020-07-23 21:58 ` Waiman Long [this message]
2020-07-24 8:16 ` Will Deacon
2020-07-24 19:10 ` Waiman Long
2020-07-25 3:02 ` Waiman Long
2020-07-25 17:26 ` Peter Zijlstra
2020-07-25 17:36 ` Waiman Long
2020-07-23 14:09 ` Nicholas Piggin
2020-07-06 4:35 ` [PATCH v3 6/6] powerpc/qspinlock: optimised atomic_try_cmpxchg_lock that adds the lock hint Nicholas Piggin
2020-07-06 18:39 ` [PATCH v3 0/6] powerpc: queued spinlocks and rwlocks Waiman Long
2020-07-07 5:57 ` Nicholas Piggin
2020-07-08 3:33 ` Waiman Long
2020-07-08 5:10 ` Nicholas Piggin
2020-07-08 23:50 ` Waiman Long
2020-07-08 23:58 ` Waiman Long
2020-07-08 8:32 ` Peter Zijlstra
2020-07-08 23:53 ` Waiman Long
2020-07-08 8:41 ` Peter Zijlstra
2020-07-08 23:54 ` Waiman Long
2020-07-09 8:31 ` Peter Zijlstra
2020-07-21 11:20 ` Nicholas Piggin
2020-07-21 11:08 ` Nicholas Piggin
2020-07-21 14:36 ` Waiman Long
2020-07-23 13:30 ` Nicholas Piggin
2020-07-23 14:29 ` Waiman Long
2020-07-23 16:12 ` Nicholas Piggin
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