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nvme: only consider exit latency when choosing useful non-op power states

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When a NVMe is in non-op states, the latency is exlat.
The latency will be enlat + exlat only when the NVMe tries to transit
from operational state right atfer it begins to transit to
non-operational state, which should be a rare case.

Therefore, as Andy Lutomirski suggests, use exlat only when deciding power
states to trainsit to.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
This commit is contained in:
Kai-Heng Feng 2017-06-07 15:25:42 +08:00 committed by Christoph Hellwig
parent 24b7f0592f
commit da87591bea
1 changed files with 9 additions and 6 deletions
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15
drivers/nvme/host/core.c
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@ -1342,7 +1342,7 @@ static void nvme_configure_apst(struct nvme_ctrl *ctrl)
* transitioning between power states. Therefore, when running
* in any given state, we will enter the next lower-power
* non-operational state after waiting 50 * (enlat + exlat)
* microseconds, as long as that state's total latency is under
* microseconds, as long as that state's exit latency is under
* the requested maximum latency.
*
* We will not autonomously enter any non-operational state for
@ -1387,7 +1387,7 @@ static void nvme_configure_apst(struct nvme_ctrl *ctrl)
* lowest-power state, not the number of states.
*/
for (state = (int)ctrl->npss; state >= 0; state--) {
u64 total_latency_us, transition_ms;
u64 total_latency_us, exit_latency_us, transition_ms;
if (target)
table->entries[state] = target;
@ -1408,12 +1408,15 @@ static void nvme_configure_apst(struct nvme_ctrl *ctrl)
NVME_PS_FLAGS_NON_OP_STATE))
continue;
total_latency_us =
(u64)le32_to_cpu(ctrl->psd[state].entry_lat) +
+ le32_to_cpu(ctrl->psd[state].exit_lat);
if (total_latency_us > ctrl->ps_max_latency_us)
exit_latency_us =
(u64)le32_to_cpu(ctrl->psd[state].exit_lat);
if (exit_latency_us > ctrl->ps_max_latency_us)
continue;
total_latency_us =
exit_latency_us +
le32_to_cpu(ctrl->psd[state].entry_lat);
/*
* This state is good. Use it as the APST idle
* target for higher power states.