linux-kernel - Re: [PATCH v2 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Date:   Wed, 3 Apr 2019 13:40:19 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alex Kogan <alex.kogan@...cle.com>, linux@...linux.org.uk,
        mingo@...hat.com, will.deacon@....com, arnd@...db.de,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, bp@...en8.de,
        hpa@...or.com, x86@...nel.org, steven.sistare@...cle.com,
        daniel.m.jordan@...cle.com, dave.dice@...cle.com,
        rahul.x.yadav@...cle.com
Subject: Re: [PATCH v2 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

On 04/03/2019 01:16 PM, Peter Zijlstra wrote:
> On Wed, Apr 03, 2019 at 12:33:20PM -0400, Waiman Long wrote:
>> static inline void queued_spin_lock_slowpath(struct qspinlock *lock, u32
>> val)
>> {
>>         if (static_branch_unlikely(&use_numa_spinlock))
>>                 numa_queued_spin_lock_slowpath(lock, val);
>>         else   
>>                 native_queued_spin_lock_slowpath(lock, val);
>> }
> That's horrible for the exact reason you state.
>
>> Alternatively, we can also call numa_queued_spin_lock_slowpath() in
>> native_queued_spin_lock_slowpath() if we don't want to increase the code
>> size of spinlock call sites.
> Yeah, still don't much like that though, we're littering the fast path
> of that slow path with all sorts of crap.

Yes, I know it is less than ideal, but that is probably the only option
if we don't have static_call or paravirt. On the other hand, I am
perfectly fine with making CNA depends on PARAVIRT_SPINLOCKS for now
until static_call is available.

Cheers,
Longman