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Subject[PATCH v9 0/4] block layer runtime pm
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In August 2010, Jens and Alan discussed about "Runtime PM and the block
layer". http://marc.info/?t=128259108400001&r=1&w=2
And then Alan has given a detailed implementation guide:
http://marc.info/?l=linux-scsi&m=133727953625963&w=2

To test:
# ls -l /sys/block/sda
/sys/devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0/0:0:0:0/block/sda

# echo 10000 > /sys/devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0/0:0:0:0/power/autosuspend_delay_ms
# echo auto > /sys/devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0/0:0:0:0/power/control
Then you'll see sda is suspended after 10secs idle.

[ 1767.680192] sd 2:0:0:0: [sda] Synchronizing SCSI cache
[ 1767.680317] sd 2:0:0:0: [sda] Stopping disk

And if you do some IO, it will resume immediately.
[ 1791.052438] sd 2:0:0:0: [sda] Starting disk

For test, I often set the autosuspend time to 1 second. If you are using
a GUI, the 10 seconds delay may be too long that the disk can not enter
runtime suspended state.

Note that sd's runtime suspend callback will dump some kernel messages
and the syslog daemon will write kernel message to /var/log/messages,
making the disk instantly resume after suspended. So for test, the
syslog daemon should better be temporarily stopped.

A git repo for it, on top of libata/upstream, scsi/for-next and
block/for-next:
https://github.com/aaronlu/linux.git blockpm

v9:
- No need to mark last busy and autosuspend in blk_pm_runtime_init as
suggested by Alan Stern;
- mark last busy in blk_runtime_post_suspend if driver failed to runtime
suspend the device, so that PM core can try to autosuspend it some
time later;
- Update scsi bus layer runtime callback to handle scsi devices which
use request based runtime PM and which don't.

v8:
- Set default autosuspend delay to -1 to avoid suspend till an updated
value is set as suggested by Alan Stern;
- Always check the dev field of the queue structure, as it is incorrect
and meaningless to do any operation on devices that do not use block
layer runtime PM as reminded by Alan Stern;
- Update scsi bus level runtime PM callback to take care of scsi devices
that use block layer runtime PM and that don't.

v7:
- Add kernel doc for block layer runtime PM API as suggested by
Alan Stern;

- Add back check for q->dev, as that serves as a flag if driver
is using block layer runtime PM;

- Do not auto suspend when last request is finished, as that's a hot
path and auto suspend is not a trivial function. Instead, mark last
busy in pre_suspend so that runtim PM core will retry suspend some
time later to solve the 1st problem demostrated in v6, suggested by
Alan Stern.

- Move block layer runtime PM strtegy functions to where they are
needed instead of in include/linux/blkdev.h as suggested by Alan
Stern since clients of block layer do not need to know those
functions.

v6:
Take over from Lin Ming.

- Instead of put the device into autosuspend state in
blk_post_runtime_suspend, do it when the last request is finished.
This can also solve the problem illustrated below:

thread A thread B
|suspend timer expired |
| ... ... |a new request comes in,
| ... ... |blk_pm_add_request
| ... ... |skip request_resume due to
| ... ... |q->status is still RPM_ACTIVE
| rpm_suspend | ... ...
| scsi_runtime_suspend | ... ...
| blk_pre_runtime_suspend | ... ...
| return -EBUSY due to nr_pending | ... ...
| rpm_suspend done | ... ...
| | blk_pm_put_request, mark last busy

But no more trigger point, and the device will stay at RPM_ACTIVE state.
Run pm_runtime_autosuspend after the last request is finished solved
this problem.

- Requests which have the REQ_PM flag should not involve nr_pending
counting, or we may lose the condition to resume the device:
Suppose queue is active and nr_pending is 0. Then a REQ_PM request
comes and nr_pending will be increased to 1, but since the request has
REQ_PM flag, it will not cause resume. Before it is finished, a normal
request comes in, and since nr_pending is 1 now, it will not trigger
the resume of the device either. Bug.

- Do not quiesce the device in scsi bus level runtime suspend callback.
Since the only reason the device is to be runtime suspended is due to
no more requests pending for it, quiesce it is pointless.

- Remove scsi_autopm_* from sd_check_events as we are request driven.

- Call blk_pm_runtime_init in scsi_sysfs_initialize_dev, so that we do
not need to check queue's device in blk_pm_add/put_request.

- Do not mark last busy and initiate an autosuspend for the device in
blk_pm_runtime_init function.

- Do not mark last busy and initiate an autosuspend for the device in
block_post_runtime_resume, as when the request that triggered the
resume finished, the blk_pm_put_request will mark last busy and
initiate an autosuspend.

v5:
- rename scsi_execute_req to scsi_execute_req_flags
and wrap scsi_execute_req around it.
- add detail function descriptions in patch 2 log
- define static helper functions to do runtime pm work on block layer
and put the definitions inside a #ifdef block

v4:
- add CONFIG_PM_RUNTIME check
- update queue runtime pm status after system resume
- use pm_runtime_autosuspend instead of pm_request_autosuspend in scsi_runtime_idle
- always count PM request

v3:
- remove block layer suspend/resume callbacks
- add block layer runtime pm helper functions

v2:
- remove queue idle timer, use runtime pm core's auto suspend

Lin Ming (4):
block: add a flag to identify PM request
block: add runtime pm helpers
block: implement runtime pm strategy
sd: change to auto suspend mode

block/blk-core.c | 183 +++++++++++++++++++++++++++++++++++++++++++++
block/elevator.c | 26 +++++++
drivers/scsi/scsi_lib.c | 9 +--
drivers/scsi/scsi_pm.c | 79 +++++++++++++++----
drivers/scsi/sd.c | 22 ++----
include/linux/blk_types.h | 2 +
include/linux/blkdev.h | 27 +++++++
include/scsi/scsi_device.h | 16 +++-
8 files changed, 326 insertions(+), 38 deletions(-)

--
1.8.1



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