linux-kernel - Re: [PATCH RFC 10/12] x86/pvticketlock: keep count of blocked cpus

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Message-ID: <20100803094451.GA5556@amd>
Date:	Tue, 3 Aug 2010 19:44:51 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Jan Beulich <JBeulich@...ell.com>, Avi Kivity <avi@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: [PATCH RFC 10/12] x86/pvticketlock: keep count of blocked cpus

On Tue, Aug 03, 2010 at 10:32:40AM +0200, Peter Zijlstra wrote:
> On Fri, 2010-07-16 at 18:03 -0700, Jeremy Fitzhardinge wrote:
> > @@ -26,6 +26,9 @@ typedef struct arch_spinlock {
> >                         __ticket_t head, tail;
> >                 } tickets;
> >         };
> > +#ifdef CONFIG_PARAVIRT_SPINLOCKS
> > +       __ticket_t waiting;
> > +#endif
> >  } arch_spinlock_t; 
> 
> This bloats spinlock_t from u32 to u64 on most distro configs I think,
> since they'll have NR_CPUS=4096 or something large like that and
> probably also want to have this PARAVIRT_SPINLOCKS thing.

Which sucks for carefully packed data structures like dentry.
They'll tend to grow by 8 bytes per spinlock.
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