On Wed, Jun 22, 2016 at 9:36 PM, Shreyas B. Prabhu
<shre...@linux.vnet.ibm.com> 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?
> Reported-by: Anton Blanchard <an...@samba.org>
> Signed-off-by: Shreyas B. Prabhu <shre...@linux.vnet.ibm.com>
> ---
> 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
>
> --
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