Messages in this thread | | | Subject | Re: [PATCH v5 1/3] sched: Stop nohz stats when decayed | From | Valentin Schneider <> | Date | Fri, 16 Feb 2018 19:23:45 +0000 |
| |
On 02/16/2018 05:02 PM, Vincent Guittot wrote: > On 16 February 2018 at 13:53, Valentin Schneider > <valentin.schneider@arm.com> wrote: >> On 02/14/2018 03:26 PM, Vincent Guittot wrote: >>> Stopped the periodic update of blocked load when all idle CPUs have fully >>> decayed. We introduce a new nohz.has_blocked that reflect if some idle >>> CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked >>> is set everytime that a Idle CPU can have blocked load and it is then clear >>> when no more blocked load has been detected during an update. We don't need >>> atomic operation but only to make cure of the right ordering when updating >>> nohz.idle_cpus_mask and nohz.has_blocked. >>> >>> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> >>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> >>> --- >>> kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- >>> kernel/sched/sched.h | 1 + >>> 2 files changed, 102 insertions(+), 21 deletions(-) >>> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>> index 7af1fa9..5a6835e 100644 >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> >>> [...] >>> >>> -static void update_nohz_stats(struct rq *rq) >>> +static bool update_nohz_stats(struct rq *rq) >>> { >>> #ifdef CONFIG_NO_HZ_COMMON >>> unsigned int cpu = rq->cpu; >>> >>> + if (!rq->has_blocked_load) >>> + return false; >>> + >>> if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask)) >>> - return; >>> + return false; >>> >>> if (!time_after(jiffies, rq->last_blocked_load_update_tick)) >>> - return; >>> + return true; >>> >>> update_blocked_averages(cpu); >>> + >>> + return rq->has_blocked_load; >>> +#else >>> + return false; >>> #endif >>> } >>> >> >> (Wrongly thought that this bit was in a different patch, comment should have >> been squashed in previous reply...) >> >> I've been thinking about this, and it's a messy one if we want to >> skip CPUs in idle_balance() / clear the nohz.has_blocked_flag. >> >> AFAICT, the rq->has_blocked_load flag can be wrongly cleared: if we're >> calling update_nohz_stats() for CPU A, but CPU A got out/in of >> idle really quickly in that same timeframe, I'm not sure you can guarantee >> the clearing of rq->has_blocked_load done in update_blocked_averages() will >> always end up in memory before the setting of the flag in >> nohz_balance_enter_idle(). > > Not sure it's a problem in this case because the clear done in > update_blocked_averages() only happens if there is no load on the rq > and new load can't be added in the mean time >
You're right, and that's why there's that comment: >> /* >> * Can be set safely without rq->lock held >> * If a clear happens, it will have evaluated last additions because >> * rq->lock is held during the check and the clear >> */ >> rq->has_blocked_load = 1;
Even though it's clearly written there my brain wouldn't process the fact that the flag is cleared with the rq lock held. So yeah, we can't wrongly clear rq->has_blocked_load. The only mishap that can happen is that it is re-raised even though we just went though an update_nohz_stats(), which would lead to a useless stats update in the future, but that's not as bad.
| |