On Wed, 2006-10-04 at 09:30 -0700, Andrew Morton wrote:
> > I have measured the cache miss with tool. So it is not just my theory.
> >
>
> And what did that tool tell you?
I am using emon. Measuring a 20 second stretch of tbench run saw the L2
cache miss go from 14 million to 25 million on each of the cpu core.
>
> Please don't just ignore my questions. *why* are we getting a cache miss
> rate on that integer which is causing measurable performance changes? If
> we're reading it that frequently then the variable should be in cache(!).
>
The point is valid, __warn_once should be in cache, unless something
evicts it. What I have found so far is with patch by Andrew and Leonid
that avoid looking up the __warn_once integer, the cache miss rate is
reduced to the level before.
> Again: do you know which callsite is causing the problem? I assume one of
> the ones in softirq.c? Do you know what the cache miss frequency is? etc.
>
Unfortunately emon does not directly give the callsite. Oprofile data
shows a marked increase in time spent in do_softirq and local_bh_enable.
What I could do is to individually turn off WARN_ON_ONCE at these sites
and see if they are responsible for the cache miss. Will let you know
what I found.
Oprofile data --
Before WARN_ON_ONCE patch:
117767 thread_return
106651 local_bh_enable
83767 tcp_v4_rcv
72266 copy_user_generic_unrolled
47136 do_softirq
41100 tcp_recvmsg
39394 tcp_sendmsg
118383 thread_return
88171 copy_user_generic
..
8281 local_bh_enable
6790 do_softirq
After WARN_ON_ONCE patch:
117767 thread_return
106651 local_bh_enable
83767 tcp_v4_rcv
72266 copy_user_generic_unrolled
47136 do_softirq
Tim
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