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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) <peterz@infradead.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Link: https://lore.kernel.org/r/20200716051130.4359-2-leo.yan@linaro.org
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Peter Zijlstra 2020-07-16 13:11:24 +08:00 committed by Will Deacon
parent 539707caa1
commit 1b86abc1c6
2 changed files with 41 additions and 28 deletions
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28
include/linux/sched_clock.h
View File

@ -6,6 +6,34 @@
#define LINUX_SCHED_CLOCK
#ifdef CONFIG_GENERIC_SCHED_CLOCK
/**
* 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;
};
extern struct clock_read_data *sched_clock_read_begin(unsigned int *seq);
extern int sched_clock_read_retry(unsigned int seq);
extern void generic_sched_clock_init(void);
extern void sched_clock_register(u64 (*read)(void), int bits,

41
kernel/time/sched_clock.c
View File

@ -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;
}