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rcu: React to callback overload by aggressively seeking quiescent states

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In default configutions, RCU currently waits at least 100 milliseconds
before asking cond_resched() and/or resched_rcu() for help seeking
quiescent states to end a grace period.  But 100 milliseconds can be
one good long time during an RCU callback flood, for example, as can
happen when user processes repeatedly open and close files in a tight
loop.  These 100-millisecond gaps in successive grace periods during a
callback flood can result in excessive numbers of callbacks piling up,
unnecessarily increasing memory footprint.

This commit therefore asks cond_resched() and/or resched_rcu() for help
as early as the first FQS scan when at least one of the CPUs has more
than 20,000 callbacks queued, a number that can be changed using the new
rcutree.qovld kernel boot parameter.  An auxiliary qovld_calc variable
is used to avoid acquisition of locks that have not yet been initialized.
Early tests indicate that this reduces the RCU-callback memory footprint
during rcutorture floods by from 50% to 4x, depending on configuration.

Reported-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reported-by: Tejun Heo <tj@kernel.org>
[ paulmck: Fix bug located by Qian Cai. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Dexuan Cui <decui@microsoft.com>
Tested-by: Qian Cai <cai@lca.pw>
This commit is contained in:
Paul E. McKenney
2019-10-30 11:56:10 -07:00
parent b5ea03709d
commit b2b00ddf19
4 changed files with 86 additions and 4 deletions
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9
Documentation/admin-guide/kernel-parameters.txt
View File

@@ -3980,6 +3980,15 @@
Set threshold of queued RCU callbacks below which
batch limiting is re-enabled.
rcutree.qovld= [KNL]
Set threshold of queued RCU callbacks beyond which
RCU's force-quiescent-state scan will aggressively
enlist help from cond_resched() and sched IPIs to
help CPUs more quickly reach quiescent states.
Set to less than zero to make this be set based
on rcutree.qhimark at boot time and to zero to
disable more aggressive help enlistment.
rcutree.rcu_idle_gp_delay= [KNL]
Set wakeup interval for idle CPUs that have
RCU callbacks (RCU_FAST_NO_HZ=y).