On 10/24/2012 05:13 PM, Marcelo Tosatti wrote:
> Improve performance of time system calls when using Linux pvclock,
> by reading time info from fixmap visible copy of pvclock data.
>
> Originally from Jeremy Fitzhardinge.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
>
> Index: vsyscall/arch/x86/vdso/vclock_gettime.c
> ===================================================================
> --- vsyscall.orig/arch/x86/vdso/vclock_gettime.c
> +++ vsyscall/arch/x86/vdso/vclock_gettime.c
> @@ -22,6 +22,7 @@
> #include <asm/hpet.h>
> #include <asm/unistd.h>
> #include <asm/io.h>
> +#include <asm/pvclock.h>
>
> #define gtod (&VVAR(vsyscall_gtod_data))
>
> @@ -62,6 +63,69 @@ static notrace cycle_t vread_hpet(void)
> return readl((const void __iomem *)fix_to_virt(VSYSCALL_HPET) + 0xf0);
> }
>
> +#ifdef CONFIG_PARAVIRT_CLOCK_VSYSCALL
> +
> +static notrace const struct pvclock_vsyscall_time_info *get_pvti(int cpu)
> +{
> + const aligned_pvti_t *pvti_base;
> + int idx = cpu / (PAGE_SIZE/PVTI_SIZE);
> + int offset = cpu % (PAGE_SIZE/PVTI_SIZE);
> +
> + BUG_ON(PVCLOCK_FIXMAP_BEGIN + idx > PVCLOCK_FIXMAP_END);
> +
> + pvti_base = (aligned_pvti_t *)__fix_to_virt(PVCLOCK_FIXMAP_BEGIN+idx);
> +
> + return &pvti_base[offset].info;
> +}
> +
Unless I am missing something, if gcc decides to not inline get_pvti,
this will break, right? I believe you need to mark that function with
__always_inline.
> +static notrace cycle_t vread_pvclock(int *mode)
> +{
> + const struct pvclock_vsyscall_time_info *pvti;
> + cycle_t ret;
> + u64 last;
> + u32 version;
> + u32 migrate_count;
> + u8 flags;
> + unsigned cpu, cpu1;
> +
> +
> + /*
> + * When looping to get a consistent (time-info, tsc) pair, we
> + * also need to deal with the possibility we can switch vcpus,
> + * so make sure we always re-fetch time-info for the current vcpu.
> + */
> + do {
> + cpu = __getcpu() & 0xfff;
Please wrap this 0xfff into something meaningful.
> + pvti = get_pvti(cpu);
> +
> + migrate_count = pvti->migrate_count;
> +
> + version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
> +
> + /*
> + * Test we're still on the cpu as well as the version.
> + * We could have been migrated just after the first
> + * vgetcpu but before fetching the version, so we
> + * wouldn't notice a version change.
> + */
> + cpu1 = __getcpu() & 0xfff;
> + } while (unlikely(cpu != cpu1 ||
> + (pvti->pvti.version & 1) ||
> + pvti->pvti.version != version ||
> + pvti->migrate_count != migrate_count));
> +
> + if (unlikely(!(flags & PVCLOCK_TSC_STABLE_BIT)))
> + *mode = VCLOCK_NONE;
> +
> + last = VVAR(vsyscall_gtod_data).clock.cycle_last;
> +
> + if (likely(ret >= last))
> + return ret;
> +
Please add a comment here referring to tsc.c, where an explanation of
this test lives. This is quite non-obvious for the non initiated.
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