Re: [PATCH] cpuidle/powernv: Fix snooze timeout
From: "Rafael J. Wysocki" <rafael@kernel.org>
Date: 2016-06-22 22:49:08
Also in:
linuxppc-dev
On Wed, Jun 22, 2016 at 9:36 PM, Shreyas B. Prabhu [off-list ref] wrote:
Snooze is a poll idle state in powernv and pseries platforms. Snooze
has a timeout so that if a cpu stays in snooze for more than target
residency of the next available idle state, then it would exit thereby
giving chance to the cpuidle governor to re-evaluate and
promote the cpu to a deeper idle state. Therefore whenever snooze exits
due to this timeout, its last_residency will be target_residency of next
deeper state.
commit e93e59ce5b85 ("cpuidle: Replace ktime_get() with local_clock()")
changed the math around last_residency calculation. Specifically, while
converting last_residency value from nanoseconds to microseconds it does
right shift by 10. Due to this, in snooze timeout exit scenarios
last_residency calculated is roughly 2.3% less than target_residency of
next available state. This pattern is picked up get_typical_interval()
in the menu governor and therefore expected_interval in menu_select() is
frequently less than the target_residency of any state but snooze.
Due to this we are entering snooze at a higher rate, thereby affecting
the single thread performance.
Since the math around last_residency is not meant to be precise, fix this
issue setting snooze timeout to 105% of target_residency of next
available idle state.
This also adds comment around why snooze timeout is necessary.Daniel, any comments?
quoted hunk ↗ jump to hunk
Reported-by: Anton Blanchard <redacted> Signed-off-by: Shreyas B. Prabhu <redacted> --- drivers/cpuidle/cpuidle-powernv.c | 14 ++++++++++++++ drivers/cpuidle/cpuidle-pseries.c | 13 +++++++++++++ 2 files changed, 27 insertions(+)diff --git a/drivers/cpuidle/cpuidle-powernv.c b/drivers/cpuidle/cpuidle-powernv.c index e12dc30..5835491 100644 --- a/drivers/cpuidle/cpuidle-powernv.c +++ b/drivers/cpuidle/cpuidle-powernv.c@@ -268,10 +268,24 @@ static int powernv_idle_probe(void) cpuidle_state_table = powernv_states; /* Device tree can indicate more idle states */ max_idle_state = powernv_add_idle_states(); + + /* + * Staying in snooze for a long period can degrade the + * perfomance of the sibling cpus. Set timeout for snooze such + * that if the cpu stays in snooze longer than target residency + * of the next available idle state then exit from snooze. This + * gives a chance to the cpuidle governor to re-evaluate and + * promote it to deeper idle states. + */ if (max_idle_state > 1) { snooze_timeout_en = true; snooze_timeout = powernv_states[1].target_residency * tb_ticks_per_usec; + /* + * Give a 5% margin since target residency related math + * is not precise in cpuidle core. + */ + snooze_timeout += snooze_timeout / 20; } } else return -ENODEV;diff --git a/drivers/cpuidle/cpuidle-pseries.c b/drivers/cpuidle/cpuidle-pseries.c index 07135e0..22de841 100644 --- a/drivers/cpuidle/cpuidle-pseries.c +++ b/drivers/cpuidle/cpuidle-pseries.c@@ -250,10 +250,23 @@ static int pseries_idle_probe(void) } else return -ENODEV; + /* + * Staying in snooze for a long period can degrade the + * perfomance of the sibling cpus. Set timeout for snooze such + * that if the cpu stays in snooze longer than target residency + * of the next available idle state then exit from snooze. This + * gives a chance to the cpuidle governor to re-evaluate and + * promote it to deeper idle states. + */ if (max_idle_state > 1) { snooze_timeout_en = true; snooze_timeout = cpuidle_state_table[1].target_residency * tb_ticks_per_usec; + /* + * Give a 5% margin since target residency related math + * is not precise in cpuidle core. + */ + snooze_timeout += snooze_timeout / 20; } return 0; } --2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-pm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html