From: Ingo Molnar <mingo@elte.hu>
Date: Sat, 8 Nov 2008 15:19:55 +0000 (+0100)
Subject: sched: improve sched_clock() performance
X-Git-Tag: v2.6.28-rc4~10^2~1
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=0d12cdd5f883f508d33b85c1bae98fa28987c8c7;p=pandora-kernel.git

sched: improve sched_clock() performance

in scheduler-intense workloads native_read_tsc() overhead accounts for
20% of the system overhead:

 659567 system_call                              41222.9375
 686796 schedule                                 435.7843
 718382 __switch_to                              665.1685
 823875 switch_mm                                4526.7857
 1883122 native_read_tsc                          55385.9412
 9761990 total                                      2.8468

this is large part due to the rdtsc_barrier() that is done before
and after reading the TSC.

But sched_clock() is not a precise clock in the GTOD sense, using such
barriers is completely pointless. So remove the barriers and only use
them in vget_cycles().

This improves lat_ctx performance by about 5%.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
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