linux-kernel - Re: [Xen-devel] Performance overhead of paravirt

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Message-ID: <4A0DB988.6000009@goop.org>
Date:	Fri, 15 May 2009 11:50:48 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jan Beulich <JBeulich@...ell.com>
CC:	Ingo Molnar <mingo@...e.hu>, Jun Nakajima <jun.nakajima@...el.com>,
	Xiaohui Xin <xiaohui.xin@...el.com>, Xin Li <xin.li@...el.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] Performance overhead of paravirt_ops on		 nativeidentified

Jan Beulich wrote:
> A patch for the pv-ops kernel would require some time. What I can give you
> right away - just for reference - are the sources we currently use in our kernel:
> attached.

Hm, I see.  Putting a call out to a pv-ops function in the ticket lock 
slow path looks pretty straightforward.  The need for an extra lock on 
the contended unlock side is a bit unfortunate; have you measured to see 
what hit that has?  Seems to me like you could avoid the problem by 
using per-cpu storage rather than stack storage (though you'd need to 
copy the per-cpu data to stack when handling a nested spinlock).

What's the thinking behind the xen_spin_adjust() stuff?

> static __always_inline void __ticket_spin_lock(raw_spinlock_t *lock) { 
> unsigned int token, count; bool free; __ticket_spin_lock_preamble; if 
> (unlikely(!free)) token = xen_spin_adjust(lock, token); do { count = 1 
> << 10; __ticket_spin_lock_body; } while (unlikely(!count) && 
> !xen_spin_wait(lock, token)); } 

How does this work?  Doesn't it always go into the slowpath loop even if 
the preamble got the lock with no contention?

    J
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