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0898a16a36
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
57 lines
1.3 KiB
C
57 lines
1.3 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef __VDSO_HELPERS_H
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#define __VDSO_HELPERS_H
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#ifndef __ASSEMBLY__
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#include <vdso/datapage.h>
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static __always_inline u32 vdso_read_begin(const struct vdso_data *vd)
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{
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u32 seq;
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while (unlikely((seq = READ_ONCE(vd->seq)) & 1))
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cpu_relax();
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smp_rmb();
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return seq;
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}
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static __always_inline u32 vdso_read_retry(const struct vdso_data *vd,
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u32 start)
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{
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u32 seq;
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smp_rmb();
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seq = READ_ONCE(vd->seq);
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return seq != start;
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}
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static __always_inline void vdso_write_begin(struct vdso_data *vd)
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{
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/*
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* WRITE_ONCE it is required otherwise the compiler can validly tear
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* updates to vd[x].seq and it is possible that the value seen by the
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* reader it is inconsistent.
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*/
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WRITE_ONCE(vd[CS_HRES_COARSE].seq, vd[CS_HRES_COARSE].seq + 1);
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WRITE_ONCE(vd[CS_RAW].seq, vd[CS_RAW].seq + 1);
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smp_wmb();
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}
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static __always_inline void vdso_write_end(struct vdso_data *vd)
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{
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smp_wmb();
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/*
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* WRITE_ONCE it is required otherwise the compiler can validly tear
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* updates to vd[x].seq and it is possible that the value seen by the
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* reader it is inconsistent.
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*/
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WRITE_ONCE(vd[CS_HRES_COARSE].seq, vd[CS_HRES_COARSE].seq + 1);
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WRITE_ONCE(vd[CS_RAW].seq, vd[CS_RAW].seq + 1);
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}
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#endif /* !__ASSEMBLY__ */
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#endif /* __VDSO_HELPERS_H */
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