[PATCH v3 1/2] x86, msr: allow rdmsr_safe_on_cpu() to schedule

Eric Dumazet
2018-03-23T21:59:24Z

I noticed high latencies caused by a daemon periodically reading
various MSR on all cpus. KASAN kernels would see ~10ms latencies
simply reading one MSR. Even without KASAN, sending IPI to CPU
in deep sleep state or blocking hard IRQ in a a long section,
then waiting for the answer can consume hundreds of usec.

Converts rdmsr_safe_on_cpu() to use a completion instead
of busy polling.

Overall daemon cpu usage was reduced by 35 %,
and latencies caused by msr_read() disappeared.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Hugh Dickins <hughd@google.com>
---
arch/x86/lib/msr-smp.c | 32 ++++++++++++++++++++++++--------
1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/arch/x86/lib/msr-smp.c b/arch/x86/lib/msr-smp.c
index 693cce0be82dffb822cecd0c7e38d2821aff896c..761ba062afdaf7f7d0603ed94ed6cc3e46b37f76 100644
--- a/arch/x86/lib/msr-smp.c
+++ b/arch/x86/lib/msr-smp.c
@@ -2,6 +2,7 @@
#include <linux/export.h>
#include <linux/preempt.h>
#include <linux/smp.h>
+#include <linux/completion.h>
#include <asm/msr.h>

static void __rdmsr_on_cpu(void *info)
@@ -143,13 +144,19 @@ void wrmsr_on_cpus(const struct cpumask *mask, u32 msr_no, struct msr *msrs)
}
EXPORT_SYMBOL(wrmsr_on_cpus);

+struct msr_info_completion {
+ struct msr_info msr;
+ struct completion done;
+};
+
/* These "safe" variants are slower and should be used when the target MSR
may not actually exist. */
static void __rdmsr_safe_on_cpu(void *info)
{
- struct msr_info *rv = info;
+ struct msr_info_completion *rv = info;

- rv->err = rdmsr_safe(rv->msr_no, &rv->reg.l, &rv->reg.h);
+ rv->msr.err = rdmsr_safe(rv->msr.msr_no, &rv->msr.reg.l, &rv->msr.reg.h);
+ complete(&rv->done);
}

static void __wrmsr_safe_on_cpu(void *info)
@@ -161,17 +168,26 @@ static void __wrmsr_safe_on_cpu(void *info)

int rdmsr_safe_on_cpu(unsigned int cpu, u32 msr_no, u32 *l, u32 *h)
{
+ struct msr_info_completion rv;
+ call_single_data_t csd = {
+ .func = __rdmsr_safe_on_cpu,
+ .info = &rv,
+ };
int err;
- struct msr_info rv;

memset(&rv, 0, sizeof(rv));
+ init_completion(&rv.done);
+ rv.msr.msr_no = msr_no;

- rv.msr_no = msr_no;
- err = smp_call_function_single(cpu, __rdmsr_safe_on_cpu, &rv, 1);
- *l = rv.reg.l;
- *h = rv.reg.h;
+ err = smp_call_function_single_async(cpu, &csd);
+ if (!err) {
+ wait_for_completion(&rv.done);
+ err = rv.msr.err;
+ }
+ *l = rv.msr.reg.l;
+ *h = rv.msr.reg.h;

- return err ? err : rv.err;
+ return err;
}
EXPORT_SYMBOL(rdmsr_safe_on_cpu);

--
2.17.0.rc0.231.g781580f067-goog