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 19:16:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
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 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.