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Message-ID: <7628453e-c418-49bb-be0a-50bc4248b7b9@redhat.com>
Date: Mon, 6 May 2024 11:05:32 -0400
From: Waiman Long <longman@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
Vernon Lovejoy <vlovejoy@...hat.com>
Subject: Re: [PATCH v2] locking/qspinlock: Save previous node & owner CPU into
mcs_spinlock
On 5/6/24 06:36, Peter Zijlstra wrote:
> On Fri, May 03, 2024 at 10:41:06PM -0400, Waiman Long wrote:
>
> Not much of a fan of this thing, but I suppose it won't hurt too much ...
I purposely doesn't touch the fast path and add code only to slow path
to make sure there shouldn't be any noticeable performance impact.
>> diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
>> index 85251d8771d9..cbe6f07dff2e 100644
>> --- a/kernel/locking/mcs_spinlock.h
>> +++ b/kernel/locking/mcs_spinlock.h
>> @@ -13,12 +13,19 @@
>> #ifndef __LINUX_MCS_SPINLOCK_H
>> #define __LINUX_MCS_SPINLOCK_H
>>
>> +/*
>> + * Save an encoded version of the current MCS lock owner CPU to the
>> + * mcs_spinlock structure of the next lock owner.
>> + */
>> +#define MCS_LOCKED (smp_processor_id() + 1)
>> +
>> #include <asm/mcs_spinlock.h>
>>
>> struct mcs_spinlock {
>> struct mcs_spinlock *next;
>> - int locked; /* 1 if lock acquired */
>> - int count; /* nesting count, see qspinlock.c */
>> + int locked; /* non-zero if lock acquired */
>> + short count; /* nesting count, see qspinlock.c */
>> + short prev_node; /* encoded previous node value */
> Strictly speaking, count shouldn't ever go much higher than 4, so I
> suppose a byte should be sufficient. That would then leave 24bit for the
> prev thing, but you'll get to use bitfields or some other horrible
> thing.
Using bit field will be more expensive. So I am deferring that until the
time it becomes necessary.
>> };
>>
>> #ifndef arch_mcs_spin_lock_contended
>> @@ -42,7 +49,7 @@ do { \
>> * unlocking.
>> */
>> #define arch_mcs_spin_unlock_contended(l) \
>> - smp_store_release((l), 1)
>> + smp_store_release((l), MCS_LOCKED)
> This leaves ARM up a creek I suppose... Also, there's but a single
> MCS_LOCKED user.
I am aware that ARM has its ownarch_mcs_spin_unlock_contended() macro
defined. That is why I define the MCS_LOCKED macro before including
asm/mcs_spinlock.h. I am planning to send an arm patch to change 1 to
MCS_LOCKED if this patch is merged to close the gap. None other arches
have arch_mcs_spin_unlock_contended() macro defined.
Cheers, Longman
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