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authorNeeraj Upadhyay <quic_neeraju@quicinc.com>2022-07-01 06:15:45 +0300
committerPaul E. McKenney <paulmck@kernel.org>2022-07-19 21:39:59 +0300
commit4f2bfd9494a072d58203600de6bedd72680e612a (patch)
tree42ca8d2af12f9e369738a400f01621adcab97e62 /Documentation
parent8f870e6eb8c0c3f9869bf3fcf9db39f86cfcea49 (diff)
downloadlinux-4f2bfd9494a072d58203600de6bedd72680e612a.tar.xz
srcu: Make expedited RCU grace periods block even less frequently
The purpose of commit 282d8998e997 ("srcu: Prevent expedited GPs and blocking readers from consuming CPU") was to prevent a long series of never-blocking expedited SRCU grace periods from blocking kernel-live-patching (KLP) progress. Although it was successful, it also resulted in excessive boot times on certain embedded workloads running under qemu with the "-bios QEMU_EFI.fd" command line. Here "excessive" means increasing the boot time up into the three-to-four minute range. This increase in boot time was due to the more than 6000 back-to-back invocations of synchronize_rcu_expedited() within the KVM host OS, which in turn resulted from qemu's emulation of a long series of MMIO accesses. Commit 640a7d37c3f4 ("srcu: Block less aggressively for expedited grace periods") did not significantly help this particular use case. Zhangfei Gao and Shameerali Kolothum Thodi did experiments varying the value of SRCU_MAX_NODELAY_PHASE with HZ=250 and with various values of non-sleeping per phase counts on a system with preemption enabled, and observed the following boot times: +──────────────────────────+────────────────+ | SRCU_MAX_NODELAY_PHASE | Boot time (s) | +──────────────────────────+────────────────+ | 100 | 30.053 | | 150 | 25.151 | | 200 | 20.704 | | 250 | 15.748 | | 500 | 11.401 | | 1000 | 11.443 | | 10000 | 11.258 | | 1000000 | 11.154 | +──────────────────────────+────────────────+ Analysis on the experiment results show additional improvements with CPU-bound delays approaching one jiffy in duration. This improvement was also seen when number of per-phase iterations were scaled to one jiffy. This commit therefore scales per-grace-period phase number of non-sleeping polls so that non-sleeping polls extend for about one jiffy. In addition, the delay-calculation call to srcu_get_delay() in srcu_gp_end() is replaced with a simple check for an expedited grace period. This change schedules callback invocation immediately after expedited grace periods complete, which results in greatly improved boot times. Testing done by Marc and Zhangfei confirms that this change recovers most of the performance degradation in boottime; for CONFIG_HZ_250 configuration, specifically, boot times improve from 3m50s to 41s on Marc's setup; and from 2m40s to ~9.7s on Zhangfei's setup. In addition to the changes to default per phase delays, this change adds 3 new kernel parameters - srcutree.srcu_max_nodelay, srcutree.srcu_max_nodelay_phase, and srcutree.srcu_retry_check_delay. This allows users to configure the srcu grace period scanning delays in order to more quickly react to additional use cases. Fixes: 640a7d37c3f4 ("srcu: Block less aggressively for expedited grace periods") Fixes: 282d8998e997 ("srcu: Prevent expedited GPs and blocking readers from consuming CPU") Reported-by: Zhangfei Gao <zhangfei.gao@linaro.org> Reported-by: yueluck <yueluck@163.com> Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com> Tested-by: Marc Zyngier <maz@kernel.org> Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org> Link: https://lore.kernel.org/all/20615615-0013-5adc-584f-2b1d5c03ebfc@linaro.org/ Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/admin-guide/kernel-parameters.txt18
1 files changed, 18 insertions, 0 deletions
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2522b11e593f..c3245baf588f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5771,6 +5771,24 @@
expediting. Set to zero to disable automatic
expediting.
+ srcutree.srcu_max_nodelay [KNL]
+ Specifies the number of no-delay instances
+ per jiffy for which the SRCU grace period
+ worker thread will be rescheduled with zero
+ delay. Beyond this limit, worker thread will
+ be rescheduled with a sleep delay of one jiffy.
+
+ srcutree.srcu_max_nodelay_phase [KNL]
+ Specifies the per-grace-period phase, number of
+ non-sleeping polls of readers. Beyond this limit,
+ grace period worker thread will be rescheduled
+ with a sleep delay of one jiffy, between each
+ rescan of the readers, for a grace period phase.
+
+ srcutree.srcu_retry_check_delay [KNL]
+ Specifies number of microseconds of non-sleeping
+ delay between each non-sleeping poll of readers.
+
srcutree.small_contention_lim [KNL]
Specifies the number of update-side contention
events per jiffy will be tolerated before