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authorSteven Rostedt (Red Hat) <rostedt@goodmis.org>2015-02-11 06:14:53 +0300
committerSteven Rostedt <rostedt@goodmis.org>2015-02-11 15:41:42 +0300
commit1e0d6714aceb770b04161fbedd7765d0e1fc27bd (patch)
treec48fa3807450ccb9790b71692eb5cce7c34f61d0 /samples
parent7215853e985a4bef1a6c14e00e89dfec84f1e457 (diff)
downloadlinux-1e0d6714aceb770b04161fbedd7765d0e1fc27bd.tar.xz
ring-buffer: Do not wake up a splice waiter when page is not full
When an application connects to the ring buffer via splice, it can only read full pages. Splice does not work with partial pages. If there is not enough data to fill a page, the splice command will either block or return -EAGAIN (if set to nonblock). Code was added where if the page is not full, to just sleep again. The problem is, it will get woken up again on the next event. That is, when something is written into the ring buffer, if there is a waiter it will wake it up. The waiter would then check the buffer, see that it still does not have enough data to fill a page and go back to sleep. To make matters worse, when the waiter goes back to sleep, it could cause another event, which would wake it back up again to see it doesn't have enough data and sleep again. This produces a tremendous overhead and fills the ring buffer with noise. For example, recording sched_switch on an idle system for 10 seconds produces 25,350,475 events!!! Create another wait queue for those waiters wanting full pages. When an event is written, it only wakes up waiters if there's a full page of data. It does not wake up the waiter if the page is not yet full. After this change, recording sched_switch on an idle system for 10 seconds produces only 800 events. Getting rid of 25,349,675 useless events (99.9969% of events!!), is something to take seriously. Cc: stable@vger.kernel.org # 3.16+ Cc: Rabin Vincent <rabin@rab.in> Fixes: e30f53aad220 "tracing: Do not busy wait in buffer splice" Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
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