Earlier commits in this series allow battery-powered systems to build
their kernels with the default-disabled CONFIG_RCU_LAZY=y Kconfig option.
This Kconfig option causes call_rcu() to delay its callbacks in order
to batch them. This means that a given RCU grace period covers more
callbacks, thus reducing the number of grace periods, in turn reducing
the amount of energy consumed, which increases battery lifetime which
can be a very good thing. This is not a subtle effect: In some important
use cases, the battery lifetime is increased by more than 10%.
This CONFIG_RCU_LAZY=y option is available only for CPUs that offload
callbacks, for example, CPUs mentioned in the rcu_nocbs kernel boot
parameter passed to kernels built with CONFIG_RCU_NOCB_CPU=y.
Delaying callbacks is normally not a problem because most callbacks do
nothing but free memory. If the system is short on memory, a shrinker
will kick all currently queued lazy callbacks out of their laziness,
thus freeing their memory in short order. Similarly, the rcu_barrier()
function, which blocks until all currently queued callbacks are invoked,
will also kick lazy callbacks, thus enabling rcu_barrier() to complete
in a timely manner.
However, there are some cases where laziness is not a good option.
For example, synchronize_rcu() invokes call_rcu(), and blocks until
the newly queued callback is invoked. It would not be a good for
synchronize_rcu() to block for ten seconds, even on an idle system.
Therefore, synchronize_rcu() invokes call_rcu_hurry() instead of
call_rcu(). The arrival of a non-lazy call_rcu_hurry() callback on a
given CPU kicks any lazy callbacks that might be already queued on that
CPU. After all, if there is going to be a grace period, all callbacks
might as well get full benefit from it.
Yes, this could be done the other way around by creating a
call_rcu_lazy(), but earlier experience with this approach and
feedback at the 2022 Linux Plumbers Conference shifted the approach
to call_rcu() being lazy with call_rcu_hurry() for the few places
where laziness is inappropriate.
And another call_rcu() instance that cannot be lazy is the one in the
scsi_eh_scmd_add() function. Leaving this instance lazy results in
unacceptably slow boot times.
Therefore, make scsi_eh_scmd_add() use call_rcu_hurry() in order to
revert to the old behavior.
[ paulmck: Apply s/call_rcu_flush/call_rcu_hurry/ feedback from Tejun Heo. ]
Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: "James E.J. Bottomley" <jejb@linux.ibm.com>
Cc: <linux-scsi@vger.kernel.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
(cherry picked from commit 54d87b0a0c19bc3f740e4cd4b87ba14ce2e4ea73)
Bug: 258241771
Signed-off-by: Joel Fernandes <joelaf@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/4909036
Reviewed-by: Vineeth Pillai <vineethrp@google.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Qais Yousef <qyousef@google.com>
Change-Id: I95bba865e582b0a12b1c09ba1f0bd4f897401c07
3751416eeb