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Date: Fri, 30 May 2008 02:52:54 -0700 (PDT) From: David Miller <davem@...emloft.net> To: pmullaney@...ell.com Cc: GHaskins@...ell.com, netdev@...r.kernel.org Subject: Re: [PATCH] net/core/sock.c remove extra wakeup From: "Patrick Mullaney" <pmullaney@...ell.com> Date: Thu, 29 May 2008 10:08:43 -0600 > We have noticed an extra wakeup of a task waiting on a udp receive > while testing with CONFIG_PREEMPT_RT. The following patch > eliminates the source of the extra wakeup by only waking a task > if it is waiting on the writable condition(the SOCK_NOSPACE > bit has been set). The patch is against 2.6.25. > > Signed-off-by: Patrick Mullaney <pmullaney@...ell.com> This looks Ok on the surface, but I'm really worried about races. If you clear that bit, another thread which set that bit and already checked the socket space, might sleep and never wake up. I think that is fundamentally why we don't try to optimize this in that way. And why is this so expensive? Once the first wakeup occurs, the subsequent attemps will merely see that the wait queue is empty and do nothing. The only cost is the read lock on the callback lock, and that is about as expensive as this new atomic operation you are adding to clear the SOCK_NOSPACE bit. So the benefit is not clear, and neither is the correctness of this change. Sorry. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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