Re: [PATCH 12/14] powerpc/64s: cpuidle no memory barrier after break from idle
From: Vaidyanathan Srinivasan <hidden>
Date: 2017-06-12 17:48:57
* Nicholas Piggin [off-list ref] [2017-06-12 09:58:33]:
A memory barrier is not required after the task wakes up, only if we clear the polling flag before waking. The case where we have work to do is the important one, so optimise for it. Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Vaidyanathan Srinivasan <redacted>
quoted hunk ↗ jump to hunk
--- drivers/cpuidle/cpuidle-powernv.c | 11 +++++++++-- drivers/cpuidle/cpuidle-pseries.c | 11 +++++++++-- 2 files changed, 18 insertions(+), 4 deletions(-)diff --git a/drivers/cpuidle/cpuidle-powernv.c b/drivers/cpuidle/cpuidle-powernv.c index 9d03326ac05e..37b0698b7193 100644 --- a/drivers/cpuidle/cpuidle-powernv.c +++ b/drivers/cpuidle/cpuidle-powernv.c@@ -59,14 +59,21 @@ static int snooze_loop(struct cpuidle_device *dev, ppc64_runlatch_off(); HMT_very_low(); while (!need_resched()) { - if (likely(snooze_timeout_en) && get_tb() > snooze_exit_time) + if (likely(snooze_timeout_en) && get_tb() > snooze_exit_time) { + /* + * Task has not woken up but we are exiting the polling + * loop anyway. Require a barrier after polling is + * cleared to order subsequent test of need_resched(). + */ + clear_thread_flag(TIF_POLLING_NRFLAG); + smp_mb(); break; + } } HMT_medium(); ppc64_runlatch_on(); clear_thread_flag(TIF_POLLING_NRFLAG); - smp_mb();
If we reach here without executing if(snooze_timeout) with the barrier+break, that means we have seen the need_resched flag and hence we can avoid the barrier. Clearing of the polling flag can be seen (take affect) later as we will exit the idle loop and re-enter anyway at this point. The caller also will check for need_resched and exit the idle loop. Actually do_idle() has a __current_set_polling() and __current_clr_polling() in the default idle loop. Do we really need to set/clear the TIF_POLLING_NRFLAG again in cpuidle driver? --Vaidy