>> this time can be reduced by 7% (113 ms) by deleting announce_cpu()
While the KERN_DEBUG output is fast, accounce_cpu() uses KERN_INFO,
which goes to the (serial) console by default.
One would expect it to drain at about 10 bits/byte / 115200 baud = 87us/byte.
I measured some vanilla printk's via KERN_INFO on this server, and they ran
at 100us/byte. I guess that isn't so far off -- an effective 100,000 baud...
announce_cpu() is actually 8 lines @ 112 bytes/line = 896 bytes, on this box.
@ 100usec/byte, that is 89ms -- about double your estimate of 50ms;
and closer to my original observation that deleting announce_cpu()
altogether saved 113ms.
I suppose somebody at 9600 baud would be less delighted -- 10x slower
would be 896 bytes * 10 bits/byte / 9600 = 933ms, yeas, almost a ful lsecond,
just for announce_cpu() output.
So I guess the question is if announce_cpu() really needs to be KERN_INFO,
or if it can be KERN_DEBUG. I think that KERN_DEBUG is sufficient,
though if it were up to me, I'd delete it entirely.
That is because the success case prints this on the console already:
[ 1.959675] x86: Booted up 4 nodes, 120 CPUs
[ 1.965202] smpboot: Total of 120 processors activated (671148.72 BogoMIPS)
Success also prints the pretty announce_cpu() list in dmesg,
and can still do so if "debug" is on the cmdline...
But most importantly, the error case would trip output
in native_cpu_up(), due to this upstream patch:
commit feef1e8ecbadf24f8e6829c935df8683cabae41b
Author: Igor Mammedov <imamm...@redhat.com>
Date: Thu Jun 5 15:42:44 2014 +0200
x86/smpboot: Log error on secondary CPU wakeup failure at ERR level
If system is running without debug level logging,
it will not log error if do_boot_cpu() failed to
wakeup AP. It may lead to silent AP bringup
failures at boot time.
Change message level to KERN_ERR to make error
visible to user as it's done on other architectures.
Signed-off-by: Igor Mammedov <imamm...@redhat.com>
Acked-by: Toshi Kani <toshi.k...@hp.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Link:
http://lkml.kernel.org/r/1401975765-22328-3-git-send-email-imamm...@redhat.com
Signed-off-by: Ingo Molnar <mi...@kernel.org>
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 2988f69..ae2fd975 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -918,7 +918,7 @@ int native_cpu_up(unsigned int cpu, struct
task_struct *tidle)
err = do_boot_cpu(apicid, cpu, tidle);
if (err) {
- pr_debug("do_boot_cpu failed %d\n", err);
+ pr_err("do_boot_cpu failed(%d) to wakeup CPU#%u\n", err, cpu);
return -EIO;
}
cheers,
Len Brown, Intel Open Source Technology Center
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