linux/include/vdso/helpers.h
Andrei Vagin 0898a16a36 lib/vdso: Add unlikely() hint into vdso_read_begin()
Place the branch with no concurrent write before the contended case.

Performance numbers for Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz
(more clock_gettime() cycles - the better):
        | before    | after
-----------------------------------
        | 150252214 | 153242367
        | 150301112 | 153324800
        | 150392773 | 153125401
        | 150373957 | 153399355
        | 150303157 | 153489417
        | 150365237 | 153494270
-----------------------------------
avg     | 150331408 | 153345935
diff %  | 2	    | 0
-----------------------------------
stdev % | 0.3	    | 0.1

Co-developed-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20191112012724.250792-2-dima@arista.com
2020-01-14 12:20:47 +01:00

57 lines
1.3 KiB
C

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __VDSO_HELPERS_H
#define __VDSO_HELPERS_H
#ifndef __ASSEMBLY__
#include <vdso/datapage.h>
static __always_inline u32 vdso_read_begin(const struct vdso_data *vd)
{
u32 seq;
while (unlikely((seq = READ_ONCE(vd->seq)) & 1))
cpu_relax();
smp_rmb();
return seq;
}
static __always_inline u32 vdso_read_retry(const struct vdso_data *vd,
u32 start)
{
u32 seq;
smp_rmb();
seq = READ_ONCE(vd->seq);
return seq != start;
}
static __always_inline void vdso_write_begin(struct vdso_data *vd)
{
/*
* WRITE_ONCE it is required otherwise the compiler can validly tear
* updates to vd[x].seq and it is possible that the value seen by the
* reader it is inconsistent.
*/
WRITE_ONCE(vd[CS_HRES_COARSE].seq, vd[CS_HRES_COARSE].seq + 1);
WRITE_ONCE(vd[CS_RAW].seq, vd[CS_RAW].seq + 1);
smp_wmb();
}
static __always_inline void vdso_write_end(struct vdso_data *vd)
{
smp_wmb();
/*
* WRITE_ONCE it is required otherwise the compiler can validly tear
* updates to vd[x].seq and it is possible that the value seen by the
* reader it is inconsistent.
*/
WRITE_ONCE(vd[CS_HRES_COARSE].seq, vd[CS_HRES_COARSE].seq + 1);
WRITE_ONCE(vd[CS_RAW].seq, vd[CS_RAW].seq + 1);
}
#endif /* !__ASSEMBLY__ */
#endif /* __VDSO_HELPERS_H */