Date: Sat, 06 Nov 2004 15:59:10 +0800 From: David Xu <davidxu@freebsd.org> To: David Schultz <das@freebsd.org> Cc: cvs-all@freebsd.org Subject: Re: cvs commit: src/sys/vm vm_zeroidle.c Message-ID: <418C844E.3030403@freebsd.org> In-Reply-To: <20041106062955.GA1986@VARK.MIT.EDU> References: <200410311932.i9VJWvmo058193@repoman.freebsd.org> <20041101045331.GP16728@cs.rice.edu> <20041101105113.GS24892@elvis.mu.org> <200411011441.33067.jhb@FreeBSD.org> <20041106062955.GA1986@VARK.MIT.EDU>
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David Schultz wrote:
>On Mon, Nov 01, 2004, John Baldwin wrote:
>
>
>>On Monday 01 November 2004 05:51 am, Alfred Perlstein wrote:
>>
>>
>>>* Alan Cox <alc@cs.rice.edu> [041031 20:53] wrote:
>>>
>>>
>>>>On Sun, Oct 31, 2004 at 07:13:17PM -0800, Alfred Perlstein wrote:
>>>>
>>>>
>>>>>* Alan Cox <alc@FreeBSD.org> [041031 11:33] wrote:
>>>>>
>>>>>
>>>>>>alc 2004-10-31 19:32:57 UTC
>>>>>>
>>>>>> FreeBSD src repository
>>>>>>
>>>>>> Modified files:
>>>>>> sys/vm vm_zeroidle.c
>>>>>> Log:
>>>>>> Introduce a Boolean variable wakeup_needed to avoid repeated,
>>>>>>unnecessary calls to wakeup() by vm_page_zero_idle_wakeup().
>>>>>>
>>>>>> Revision Changes Path
>>>>>> 1.31 +9 -2 src/sys/vm/vm_zeroidle.c
>>>>>>
>>>>>>
>>>>>Why not switch to a cv?
>>>>>
>>>>>
>>>>Calling cv_signal repeatedly would be no better than calling wakeup()
>>>>repeatedly. Either way, a Boolean variable is desirable to prevent
>>>>unnecessary calls.
>>>>
>>>>
>>>Yah, I figured there would be something in the cv code to optimize
>>>the "no waiters" case.
>>>
>>>
>>There is, though sometimes it might think there are waiters when there
>>actually aren't any.
>>
>>
>
>It doesn't look very optimized:
>
> void
> cv_signal(struct cv *cvp)
> {
>
> sleepq_lock(cvp);
> ^^^^^^^^^^^^^^^^^
> if (cvp->cv_waiters > 0) {
> [...]
> } else
> sleepq_release(cvp);
>
>The mutex associated with the condition variable will be held on
>entry to both cv_wait() and cv_signal(), so why is the sleepqueue
>locked in the no waiters case?
>
>
>
>
cv_wait and cv_signal have to pass a serialization point,
the serialization point is condition variable internal lock,
before testing cvp_waiters, the internal lock should be
locked, otherwise you should be ready to hit the race
condition. It is legal to call cv_signal() without a locked
mutex.
David Xu
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