Messages in this thread Patch in this message | | | Date | Tue, 15 Jan 2008 17:50:39 -0500 | From | Pete Wyckoff <> | Subject | Re: nosmp/maxcpus=0 or 1 -> TSC unstable |
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dean@arctic.org wrote on Sat, 12 Jan 2008 11:48 -0800: > if i boot an x86 64-bit 2.6.24-rc7 kernel with nosmp, maxcpus=0 or 1 it > still disables TSC :) > > Marking TSC unstable due to TSCs unsynchronized > > this is an opteron 2xx box which does have two cpus and no clock-divide in > halt or cpufreq enabled so TSC should be fine with only one cpu. > > pretty sure this is the culprit is that num_possible_cpus() > 1, which > would mean cpu_possible_map contains the second cpu... but i'm not quite > sure what the right fix is... or perhaps this is all intended.
We've seen the same problem. We use gettimeofday() for timing of network-ish operations on the order of 10-50 us. But not having the TSC makes gettimeofday() itself very slow, on the order of 30 us.
Here's what we've been using for quite a few kernel versions. I've not tried to submit it for fear that it could break some other scenario, as you suggest. Although in hotplug scenarios, this function unsynchronized_tsc() should get rerun and disable TSC if more processors arrive.
At least count this as a "me too".
-- Pete
From 0cdcd494bc0e27f49438bc2fc72fd3823629802b Mon Sep 17 00:00:00 2001 From: Pete Wyckoff <pw@osc.edu> Date: Tue, 15 Jan 2008 17:42:28 -0500 Subject: [PATCH] use tsc on 1 cpu smp
Use num_online_cpus() instead of num_present_cpus() as the parameter to check when deciding if TSC is good enough. Thus explicitly booting with maxcpus=1 will let us use the TSC even on a dual-processor machine. This helps reduce gettimeofday overheads on our dual Opteron nodes immensely (30 us vs 0.5 us).
Signed-off-by: Pete Wyckoff <pw@osc.edu> --- arch/x86/kernel/tsc_64.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/arch/x86/kernel/tsc_64.c b/arch/x86/kernel/tsc_64.c index 9c70af4..5f2e91f 100644 --- a/arch/x86/kernel/tsc_64.c +++ b/arch/x86/kernel/tsc_64.c @@ -235,7 +235,7 @@ __cpuinit int unsynchronized_tsc(void) } /* Assume multi socket systems are not synchronized */ - return num_present_cpus() > 1; + return num_online_cpus() > 1; } int __init notsc_setup(char *s) -- 1.5.3.7
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