On Mon, Feb 13, 2012 at 04:45:59PM +0100, Igor Mammedov wrote:
> Instead of hunting misterious stalls/hungs all over the kernel when
> overflow occurs at pvclock.c:pvclock_get_nsec_offset
>
> u64 delta = native_read_tsc() - shadow->tsc_timestamp;
>
> and introducing hooks when places of unexpected access found, pv_clock
> should be initialized for the calling cpu if overflow condition is detected.
>
> Signed-off-by: Igor Mammedov <imammedo@xxxxxxxxxx>
Igor,
I disagree. This is fixing the symptom not the root cause. Additionally,
Xen also uses pvclock_clocksource_read.
How about adding a BUG_ON to detect the overflow, this way hunting for
the problem is not necessary.
> arch/x86/kernel/pvclock.c | 18 +++++++++++++++---
> 1 files changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
> index 42eb330..b486756 100644
> --- a/arch/x86/kernel/pvclock.c
> +++ b/arch/x86/kernel/pvclock.c
> @@ -41,9 +41,14 @@ void pvclock_set_flags(u8 flags)
> valid_flags = flags;
> }
>
> -static u64 pvclock_get_nsec_offset(struct pvclock_shadow_time *shadow)
> +static u64 pvclock_get_nsec_offset(struct pvclock_shadow_time *shadow,
> + bool *overflow)
> {
> - u64 delta = native_read_tsc() - shadow->tsc_timestamp;
> + u64 delta;
> + u64 tsc = native_read_tsc();
> + u64 shadow_timestamp = shadow->tsc_timestamp;
> + *overflow = tsc < shadow_timestamp;
> + delta = tsc - shadow_timestamp;
> return pvclock_scale_delta(delta, shadow->tsc_to_nsec_mul,
> shadow->tsc_shift);
> }
> @@ -94,12 +99,19 @@ cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
> unsigned version;
> cycle_t ret, offset;
> u64 last;
> + bool overflow;
>
> do {
> version = pvclock_get_time_values(&shadow, src);
> barrier();
> - offset = pvclock_get_nsec_offset(&shadow);
> + offset = pvclock_get_nsec_offset(&shadow, &overflow);
> ret = shadow.system_timestamp + offset;
> + if (unlikely(overflow)) {
> + memset(src, 0, sizeof(*src));
> + barrier();
> + x86_cpuinit.early_percpu_clock_init();
> + continue;
> + }
> barrier();
> } while (version != src->version);
>
> --
> 1.7.7.6
>
> --
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