| Message ID | 20240726201332.626395-2-ankur.a.arora@oracle.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Enable haltpoll on arm64 | expand |
On Fri, 26 Jul 2024, Ankur Arora wrote: > 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; loop_count is only used in the while loop below. So the declaration could be placed below the while. > > 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; > } Looks ok otherwise Reviewed-by: Christoph Lameter <cl@linux.com>
> Subject: [PATCH v6 01/10] cpuidle/poll_state: poll via smp_cond_load_relaxed() > > From: Mihai Carabas <mihai.carabas@oracle.com> > > The inner loop in poll_idle() polls up to POLL_IDLE_RELAX_COUNT times, > checking to see if the thread has the TIF_NEED_RESCHED bit set. 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 <peterz@infradead.org> > Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com> > Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com> > --- > 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 <misono.tomohiro@fujitsu.com> Regards, Tomohiro
Christoph Lameter (Ampere) <cl@gentwo.org> writes: > On Fri, 26 Jul 2024, Ankur Arora wrote: > >> 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; > > loop_count is only used in the while loop below. So the declaration could be > placed below the while. That's a good idea. Will fix. >> >> 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; >> } > > Looks ok otherwise > > Reviewed-by: Christoph Lameter <cl@linux.com> Thanks for the review. -- ankur
Tomohiro Misono (Fujitsu) <misono.tomohiro@fujitsu.com> writes: >> Subject: [PATCH v6 01/10] cpuidle/poll_state: poll via smp_cond_load_relaxed() >> >> From: Mihai Carabas <mihai.carabas@oracle.com> >> >> The inner loop in poll_idle() polls up to POLL_IDLE_RELAX_COUNT times, >> checking to see if the thread has the TIF_NEED_RESCHED bit set. 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 <peterz@infradead.org> >> Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com> >> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com> >> --- >> 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 <misono.tomohiro@fujitsu.com> Thanks! -- ankur
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();