From 1b86abc1c645ad5c9c7bf70910cb3ce73939d2d7 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 16 Jul 2020 13:11:24 +0800 Subject: sched_clock: Expose struct clock_read_data In order to support perf_event_mmap_page::cap_time features, an architecture needs, aside from a userspace readable counter register, to expose the exact clock data so that userspace can convert the counter register into a correct timestamp. Provide struct clock_read_data and two (seqcount) helpers so that architectures (arm64 in specific) can expose the numbers to userspace. Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Leo Yan Link: https://lore.kernel.org/r/20200716051130.4359-2-leo.yan@linaro.org Signed-off-by: Will Deacon --- kernel/time/sched_clock.c | 41 +++++++++++++---------------------------- 1 file changed, 13 insertions(+), 28 deletions(-) (limited to 'kernel/time') diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c index fa3f800d7d76..0acaadc3156c 100644 --- a/kernel/time/sched_clock.c +++ b/kernel/time/sched_clock.c @@ -19,31 +19,6 @@ #include "timekeeping.h" -/** - * struct clock_read_data - data required to read from sched_clock() - * - * @epoch_ns: sched_clock() value at last update - * @epoch_cyc: Clock cycle value at last update. - * @sched_clock_mask: Bitmask for two's complement subtraction of non 64bit - * clocks. - * @read_sched_clock: Current clock source (or dummy source when suspended). - * @mult: Multipler for scaled math conversion. - * @shift: Shift value for scaled math conversion. - * - * Care must be taken when updating this structure; it is read by - * some very hot code paths. It occupies <=40 bytes and, when combined - * with the seqcount used to synchronize access, comfortably fits into - * a 64 byte cache line. - */ -struct clock_read_data { - u64 epoch_ns; - u64 epoch_cyc; - u64 sched_clock_mask; - u64 (*read_sched_clock)(void); - u32 mult; - u32 shift; -}; - /** * struct clock_data - all data needed for sched_clock() (including * registration of a new clock source) @@ -93,6 +68,17 @@ static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift) return (cyc * mult) >> shift; } +struct clock_read_data *sched_clock_read_begin(unsigned int *seq) +{ + *seq = raw_read_seqcount(&cd.seq); + return cd.read_data + (*seq & 1); +} + +int sched_clock_read_retry(unsigned int seq) +{ + return read_seqcount_retry(&cd.seq, seq); +} + unsigned long long notrace sched_clock(void) { u64 cyc, res; @@ -100,13 +86,12 @@ unsigned long long notrace sched_clock(void) struct clock_read_data *rd; do { - seq = raw_read_seqcount(&cd.seq); - rd = cd.read_data + (seq & 1); + rd = sched_clock_read_begin(&seq); cyc = (rd->read_sched_clock() - rd->epoch_cyc) & rd->sched_clock_mask; res = rd->epoch_ns + cyc_to_ns(cyc, rd->mult, rd->shift); - } while (read_seqcount_retry(&cd.seq, seq)); + } while (sched_clock_read_retry(seq)); return res; } -- cgit v1.2.3-59-g8ed1b From aadd6e5caaacd6feca9691ba30536e7de5a7d152 Mon Sep 17 00:00:00 2001 From: "Ahmed S. Darwish" Date: Thu, 16 Jul 2020 13:11:25 +0800 Subject: time/sched_clock: Use raw_read_seqcount_latch() sched_clock uses seqcount_t latching to switch between two storage places protected by the sequence counter. This allows it to have interruptible, NMI-safe, seqcount_t write side critical sections. Since 7fc26327b756 ("seqlock: Introduce raw_read_seqcount_latch()"), raw_read_seqcount_latch() became the standardized way for seqcount_t latch read paths. Due to the dependent load, it also has one read memory barrier less than the currently used raw_read_seqcount() API. Use raw_read_seqcount_latch() for the seqcount_t latch read path. Signed-off-by: Ahmed S. Darwish Signed-off-by: Leo Yan Link: https://lkml.kernel.org/r/20200625085745.GD117543@hirez.programming.kicks-ass.net Link: https://lkml.kernel.org/r/20200715092345.GA231464@debian-buster-darwi.lab.linutronix.de Link: https://lore.kernel.org/r/20200716051130.4359-3-leo.yan@linaro.org References: 1809bfa44e10 ("timers, sched/clock: Avoid deadlock during read from NMI") Signed-off-by: Will Deacon --- kernel/time/sched_clock.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'kernel/time') diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c index 0acaadc3156c..0deaf4b79fb4 100644 --- a/kernel/time/sched_clock.c +++ b/kernel/time/sched_clock.c @@ -70,7 +70,7 @@ static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift) struct clock_read_data *sched_clock_read_begin(unsigned int *seq) { - *seq = raw_read_seqcount(&cd.seq); + *seq = raw_read_seqcount_latch(&cd.seq); return cd.read_data + (*seq & 1); } -- cgit v1.2.3-59-g8ed1b