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The >> loop exits once the condition is met, or if the poll time limit has >> been exceeded. >> >> To minimize the number of instructions executed each iteration, the >> time check is done only infrequently (once every POLL_IDLE_RELAX_COUNT >> iterations). In addition, each loop iteration executes cpu_relax() >> which on certain platforms provides a hint to the pipeline that the >> loop is busy-waiting, thus allowing the processor to reduce power >> consumption. >> >> However, cpu_relax() is defined optimally only on x86. On arm64, for >> instance, it is implemented as a YIELD which only serves a hint to the >> CPU that it prioritize a different hardware thread if one is available. >> arm64, however, does expose a more optimal polling mechanism via >> smp_cond_load_relaxed() which uses LDXR, WFE to wait until a store >> to a specified region. >> >> So restructure the loop, folding both checks in smp_cond_load_relaxed(). >> Also, move the time check to the head of the loop allowing it to exit >> straight-away once TIF_NEED_RESCHED is set. >> >> Suggested-by: Peter Zijlstra >> Signed-off-by: Mihai Carabas >> Signed-off-by: Ankur Arora >> --- >> drivers/cpuidle/poll_state.c | 10 +++++----- >> 1 file changed, 5 insertions(+), 5 deletions(-) >> >> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c >> index 9b6d90a72601..532e4ed19e0f 100644 >> --- a/drivers/cpuidle/poll_state.c >> +++ b/drivers/cpuidle/poll_state.c >> @@ -21,21 +21,21 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev, >> >> raw_local_irq_enable(); >> if (!current_set_polling_and_test()) { >> - unsigned int loop_count = 0; >> + unsigned int loop_count; >> u64 limit; >> >> limit = cpuidle_poll_time(drv, dev); >> >> while (!need_resched()) { >> - cpu_relax(); >> - if (loop_count++ < POLL_IDLE_RELAX_COUNT) >> - continue; >> - >> loop_count = 0; >> if (local_clock_noinstr() - time_start > limit) { >> dev->poll_time_limit = true; >> break; >> } >> + >> + smp_cond_load_relaxed(¤t_thread_info()->flags, >> + VAL & _TIF_NEED_RESCHED || >> + loop_count++ >= POLL_IDLE_RELAX_COUNT); >> } >> } >> raw_local_irq_disable(); >> -- >> 2.43.5 > > Reviewed-by: Misono Tomohiro Thanks! -- ankur