linux-kernel - Re: [PATCH v6] x86/hpet: Reduce HPET counter read contention

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Date:   Thu, 25 Aug 2016 16:07:25 -0400
From:   Waiman Long <waiman.long@....com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, <linux-kernel@...r.kernel.org>,
        <x86@...nel.org>, Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Prarit Bhargava <prarit@...hat.com>,
        Scott J Norton <scott.norton@....com>,
        Douglas Hatch <doug.hatch@....com>,
        Randy Wright <rwright@....com>
Subject: Re: [PATCH v6] x86/hpet: Reduce HPET counter read contention

On 08/25/2016 02:50 PM, Dave Hansen wrote:
> On 08/12/2016 05:59 PM, Waiman Long wrote:
>> + * The lock and the hpet value are stored together and can be read in a
>> + * single atomic 64-bit read. It is explicitly assumed that arch_spinlock_t
>> + * is 32 bits in size.
> This requirement forces us to give up all of the goodness of lockdep.
> Is this strictly a performance optimization or is there some function
> requirement behind it as well?

Yes, it is mostly performance optimization. Using a full spinlock will 
require additional synchronization code like a memory barrier to prevent 
race between the lock and HPET value with respect to the readers.

It is a simple lock that won't have additional locks nested inside. So I 
wonder if there is any value in having the lockdep functionality for 
this lock.

Cheers,
Longman