user/sven/linux.git/kernel/time/clocksource.c, branch v6.1.56

clocksource: Suspend the watchdog temporarily when high read latency detected

2023-03-10T08:33:50Z

[ Upstream commit b7082cdfc464bf9231300605d03eebf943dda307 ] Bugs have been reported on 8 sockets x86 machines in which the TSC was wrongly disabled when the system is under heavy workload. [ 818.380354] clocksource: timekeeping watchdog on CPU336: hpet wd-wd read-back delay of 1203520ns [ 818.436160] clocksource: wd-tsc-wd read-back delay of 181880ns, clock-skew test skipped! [ 819.402962] clocksource: timekeeping watchdog on CPU338: hpet wd-wd read-back delay of 324000ns [ 819.448036] clocksource: wd-tsc-wd read-back delay of 337240ns, clock-skew test skipped! [ 819.880863] clocksource: timekeeping watchdog on CPU339: hpet read-back delay of 150280ns, attempt 3, marking unstable [ 819.936243] tsc: Marking TSC unstable due to clocksource watchdog [ 820.068173] TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'. [ 820.092382] sched_clock: Marking unstable (818769414384, 1195404998) [ 820.643627] clocksource: Checking clocksource tsc synchronization from CPU 267 to CPUs 0,4,25,70,126,430,557,564. [ 821.067990] clocksource: Switched to clocksource hpet This can be reproduced by running memory intensive 'stream' tests, or some of the stress-ng subcases such as 'ioport'. The reason for these issues is the when system is under heavy load, the read latency of the clocksources can be very high. Even lightweight TSC reads can show high latencies, and latencies are much worse for external clocksources such as HPET or the APIC PM timer. These latencies can result in false-positive clocksource-unstable determinations. These issues were initially reported by a customer running on a production system, and this problem was reproduced on several generations of Xeon servers, especially when running the stress-ng test. These Xeon servers were not production systems, but they did have the latest steppings and firmware. Given that the clocksource watchdog is a continual diagnostic check with frequency of twice a second, there is no need to rush it when the system is under heavy load. Therefore, when high clocksource read latencies are detected, suspend the watchdog timer for 5 minutes. Signed-off-by: Feng Tang Acked-by: Waiman Long Cc: John Stultz Cc: Thomas Gleixner Cc: Stephen Boyd Cc: Feng Tang Signed-off-by: Paul E. McKenney Signed-off-by: Sasha Levin

treewide: use prandom_u32_max() when possible, part 1

2022-10-11T23:42:55Z

Rather than incurring a division or requesting too many random bytes for the given range, use the prandom_u32_max() function, which only takes the minimum required bytes from the RNG and avoids divisions. This was done mechanically with this coccinelle script: @basic@ expression E; type T; identifier get_random_u32 =~ "get_random_int|prandom_u32|get_random_u32"; typedef u64; @@ ( - ((T)get_random_u32() % (E)) + prandom_u32_max(E) | - ((T)get_random_u32() & ((E) - 1)) + prandom_u32_max(E * XXX_MAKE_SURE_E_IS_POW2) | - ((u64)(E) * get_random_u32() >> 32) + prandom_u32_max(E) | - ((T)get_random_u32() & ~PAGE_MASK) + prandom_u32_max(PAGE_SIZE) ) @multi_line@ identifier get_random_u32 =~ "get_random_int|prandom_u32|get_random_u32"; identifier RAND; expression E; @@ - RAND = get_random_u32(); ... when != RAND - RAND %= (E); + RAND = prandom_u32_max(E); // Find a potential literal @literal_mask@ expression LITERAL; type T; identifier get_random_u32 =~ "get_random_int|prandom_u32|get_random_u32"; position p; @@ ((T)get_random_u32()@p & (LITERAL)) // Add one to the literal. @script:python add_one@ literal << literal_mask.LITERAL; RESULT; @@ value = None if literal.startswith('0x'): value = int(literal, 16) elif literal[0] in '123456789': value = int(literal, 10) if value is None: print("I don't know how to handle %s" % (literal)) cocci.include_match(False) elif value == 2**32 - 1 or value == 2**31 - 1 or value == 2**24 - 1 or value == 2**16 - 1 or value == 2**8 - 1: print("Skipping 0x%x for cleanup elsewhere" % (value)) cocci.include_match(False) elif value & (value + 1) != 0: print("Skipping 0x%x because it's not a power of two minus one" % (value)) cocci.include_match(False) elif literal.startswith('0x'): coccinelle.RESULT = cocci.make_expr("0x%x" % (value + 1)) else: coccinelle.RESULT = cocci.make_expr("%d" % (value + 1)) // Replace the literal mask with the calculated result. @plus_one@ expression literal_mask.LITERAL; position literal_mask.p; expression add_one.RESULT; identifier FUNC; @@ - (FUNC()@p & (LITERAL)) + prandom_u32_max(RESULT) @collapse_ret@ type T; identifier VAR; expression E; @@ { - T VAR; - VAR = (E); - return VAR; + return E; } @drop_var@ type T; identifier VAR; @@ { - T VAR; ... when != VAR } Reviewed-by: Greg Kroah-Hartman Reviewed-by: Kees Cook Reviewed-by: Yury Norov Reviewed-by: KP Singh Reviewed-by: Jan Kara # for ext4 and sbitmap Reviewed-by: Christoph Böhmwalder # for drbd Acked-by: Jakub Kicinski Acked-by: Heiko Carstens # for s390 Acked-by: Ulf Hansson # for mmc Acked-by: Darrick J. Wong # for xfs Signed-off-by: Jason A. Donenfeld

clocksource: Replace cpumask_weight() with cpumask_empty()

2022-04-10T20:30:04Z

clocksource_verify_percpu() calls cpumask_weight() to check if any bit of a given cpumask is set. This can be done more efficiently with cpumask_empty() because cpumask_empty() stops traversing the cpumask as soon as it finds first set bit, while cpumask_weight() counts all bits unconditionally. Signed-off-by: Yury Norov Signed-off-by: Thomas Gleixner Link: https://lore.kernel.org/r/20220210224933.379149-24-yury.norov@gmail.com

clocksource: Add a Kconfig option for WATCHDOG_MAX_SKEW

2022-02-02T01:35:43Z

A watchdog maximum skew of 100us may still be too small for some systems or archs. It may also be too small when some kernel debug config options are enabled. So add a new Kconfig option CLOCKSOURCE_WATCHDOG_MAX_SKEW_US to allow kernel builders to have more control on the threshold for marking clocksource as unstable. Signed-off-by: Waiman Long Signed-off-by: Paul E. McKenney

Merge tag 'bitmap-5.17-rc1' of git://github.com/norov/linux

2022-01-23T04:20:44Z

Pull bitmap updates from Yury Norov: - introduce for_each_set_bitrange() - use find_first_*_bit() instead of find_next_*_bit() where possible - unify for_each_bit() macros * tag 'bitmap-5.17-rc1' of git://github.com/norov/linux: vsprintf: rework bitmap_list_string lib: bitmap: add performance test for bitmap_print_to_pagebuf bitmap: unify find_bit operations mm/percpu: micro-optimize pcpu_is_populated() Replace for_each_*_bit_from() with for_each_*_bit() where appropriate find: micro-optimize for_each_{set,clear}_bit() include/linux: move for_each_bit() macros from bitops.h to find.h cpumask: replace cpumask_next_* with cpumask_first_* where appropriate tools: sync tools/bitmap with mother linux all: replace find_next{,_zero}_bit with find_first{,_zero}_bit where appropriate cpumask: use find_first_and_bit() lib: add find_first_and_bit() arch: remove GENERIC_FIND_FIRST_BIT entirely include: move find.h from asm_generic to linux bitops: move find_bit_*_le functions from le.h to find.h bitops: protect find_first_{,zero}_bit properly

cpumask: replace cpumask_next_* with cpumask_first_* where appropriate

2022-01-15T16:47:31Z

cpumask_first() is a more effective analogue of 'next' version if n == -1 (which means start == 0). This patch replaces 'next' with 'first' where things look trivial. There's no cpumask_first_zero() function, so create it. Signed-off-by: Yury Norov Tested-by: Wolfram Sang

clocksource: Reduce the default clocksource_watchdog() retries to 2

2021-12-01T01:22:29Z

With the previous patch, there is an extra watchdog read in each retry. Now the total number of clocksource reads is increased to 4 per iteration. In order to avoid increasing the clock skew check overhead, the default maximum number of retries is reduced from 3 to 2 to maintain the same 12 clocksource reads in the worst case. Suggested-by: Paul E. McKenney Signed-off-by: Waiman Long Signed-off-by: Paul E. McKenney

clocksource: Avoid accidental unstable marking of clocksources

2021-12-01T01:22:29Z

Since commit db3a34e17433 ("clocksource: Retry clock read if long delays detected") and commit 2e27e793e280 ("clocksource: Reduce clocksource-skew threshold"), it is found that tsc clocksource fallback to hpet can sometimes happen on both Intel and AMD systems especially when they are running stressful benchmarking workloads. Of the 23 systems tested with a v5.14 kernel, 10 of them have switched to hpet clock source during the test run. The result of falling back to hpet is a drastic reduction of performance when running benchmarks. For example, the fio performance tests can drop up to 70% whereas the iperf3 performance can drop up to 80%. 4 hpet fallbacks happened during bootup. They were: [ 8.749399] clocksource: timekeeping watchdog on CPU13: hpet read-back delay of 263750ns, attempt 4, marking unstable [ 12.044610] clocksource: timekeeping watchdog on CPU19: hpet read-back delay of 186166ns, attempt 4, marking unstable [ 17.336941] clocksource: timekeeping watchdog on CPU28: hpet read-back delay of 182291ns, attempt 4, marking unstable [ 17.518565] clocksource: timekeeping watchdog on CPU34: hpet read-back delay of 252196ns, attempt 4, marking unstable Other fallbacks happen when the systems were running stressful benchmarks. For example: [ 2685.867873] clocksource: timekeeping watchdog on CPU117: hpet read-back delay of 57269ns, attempt 4, marking unstable [46215.471228] clocksource: timekeeping watchdog on CPU8: hpet read-back delay of 61460ns, attempt 4, marking unstable Commit 2e27e793e280 ("clocksource: Reduce clocksource-skew threshold"), changed the skew margin from 100us to 50us. I think this is too small and can easily be exceeded when running some stressful workloads on a thermally stressed system. So it is switched back to 100us. Even a maximum skew margin of 100us may be too small in for some systems when booting up especially if those systems are under thermal stress. To eliminate the case that the large skew is due to the system being too busy slowing down the reading of both the watchdog and the clocksource, an extra consecutive read of watchdog clock is being done to check this. The consecutive watchdog read delay is compared against WATCHDOG_MAX_SKEW/2. If the delay exceeds the limit, we assume that the system is just too busy. A warning will be printed to the console and the clock skew check is skipped for this round. Fixes: db3a34e17433 ("clocksource: Retry clock read if long delays detected") Fixes: 2e27e793e280 ("clocksource: Reduce clocksource-skew threshold") Signed-off-by: Waiman Long Signed-off-by: Paul E. McKenney

clocksource: Replace deprecated CPU-hotplug functions.

2021-08-10T12:53:58Z

The functions get_online_cpus() and put_online_cpus() have been deprecated during the CPU hotplug rework. They map directly to cpus_read_lock() and cpus_read_unlock(). Replace deprecated CPU-hotplug functions with the official version. The behavior remains unchanged. Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Thomas Gleixner Link: https://lore.kernel.org/r/20210803141621.780504-35-bigeasy@linutronix.de

clocksource: Print deviation in nanoseconds when a clocksource becomes unstable

2021-06-22T14:53:17Z

Currently when an unstable clocksource is detected, the raw counters of that clocksource and watchdog will be printed, which can only be understood after some math calculation. So print the delta in nanoseconds as well to make it easier for humans to check the results. [ paulmck: Fix typo. ] Signed-off-by: Feng Tang Signed-off-by: Paul E. McKenney Signed-off-by: Thomas Gleixner Link: https://lore.kernel.org/r/20210527190124.440372-6-paulmck@kernel.org