[EMAIL PROTECTED] 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 <[EMAIL PROTECTED]>
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 <[EMAIL PROTECTED]>
---
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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
