linux-kernel - Re: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Date:   Tue, 16 Jul 2019 13:04:27 +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, guohanjun@...wei.com,
        jglauber@...vell.com, steven.sistare@...cle.com,
        daniel.m.jordan@...cle.com, dave.dice@...cle.com,
        rahul.x.yadav@...cle.com
Subject: Re: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

On Mon, Jul 15, 2019 at 05:30:01PM -0400, Waiman Long wrote:
> On 7/15/19 3:25 PM, Alex Kogan wrote:
> >  /*
> > - * On 64-bit architectures, the mcs_spinlock structure will be 16 bytes in
> > - * size and four of them will fit nicely in one 64-byte cacheline. For
> > - * pvqspinlock, however, we need more space for extra data. To accommodate
> > - * that, we insert two more long words to pad it up to 32 bytes. IOW, only
> > - * two of them can fit in a cacheline in this case. That is OK as it is rare
> > - * to have more than 2 levels of slowpath nesting in actual use. We don't
> > - * want to penalize pvqspinlocks to optimize for a rare case in native
> > - * qspinlocks.
> > + * On 64-bit architectures, the mcs_spinlock structure will be 20 bytes in
> > + * size. For pvqspinlock or the NUMA-aware variant, however, we need more
> > + * space for extra data. To accommodate that, we insert two more long words
> > + * to pad it up to 36 bytes.
> >   */

> The 20 bytes figure is wrong. It is actually 24 bytes for 64-bit as the
> mcs_spinlock structure is 8-byte aligned. For better cacheline
> alignment, I will like to keep mcs_spinlock to 16 bytes as before.
> Instead, you can use encode_tail() to store the CNA node pointer in
> "locked". For instance, use (encode_tail() << 1) in locked to
> distinguish it from the regular locked=1 value.

Yes, please don't bloat this. I already don't like what Waiman did for
the paravirt case, but this is horrible.