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If a CPU is running either a userspace application or a guest OS in nohz_full mode, it is possible for a system call to occur just as an RCU grace period is starting. If that CPU also has the scheduling-clock tick enabled for any reason (such as a second runnable task), and if the system was booted with rcutree.use_softirq=0, then RCU can add insult to injury by awakening that CPU's rcuc kthread, resulting in yet another task and yet more OS jitter due to switching to that task, running it, and switching back. In addition, in the common case where that system call is not of excessively long duration, awakening the rcuc task is pointless. This pointlessness is due to the fact that the CPU will enter an extended quiescent state upon returning to the userspace application or guest OS. In this case, the rcuc kthread cannot do anything that the main RCU grace-period kthread cannot do on its behalf, at least if it is given a few additional milliseconds (for example, given the time duration specified by rcutree.jiffies_till_first_fqs, give or take scheduling delays). This commit therefore adds a rcutree.nohz_full_patience_delay kernel boot parameter that specifies the grace period age (in milliseconds, rounded to jiffies) before which RCU will refrain from awakening the rcuc kthread. Preliminary experimentation suggests a value of 1000, that is, one second. Increasing rcutree.nohz_full_patience_delay will increase grace-period latency and in turn increase memory footprint, so systems with constrained memory might choose a smaller value. Systems with less-aggressive OS-jitter requirements might choose the default value of zero, which keeps the traditional immediate-wakeup behavior, thus avoiding increases in grace-period latency. [ paulmck: Apply Leonardo Bras feedback. ] Link: https://lore.kernel.org/all/20240328171949.743211-1-leobras@redhat.com/ Reported-by: Leonardo Bras <leobras@redhat.com> Suggested-by: Leonardo Bras <leobras@redhat.com> Suggested-by: Sean Christopherson <seanjc@google.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Reviewed-by: Leonardo Bras <leobras@redhat.com> |
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| .. | ||
| Kconfig | ||
| Kconfig.debug | ||
| Makefile | ||
| rcu_segcblist.c | ||
| rcu_segcblist.h | ||
| rcu.h | ||
| rcuscale.c | ||
| rcutorture.c | ||
| refscale.c | ||
| srcutiny.c | ||
| srcutree.c | ||
| sync.c | ||
| tasks.h | ||
| tiny.c | ||
| tree_exp.h | ||
| tree_nocb.h | ||
| tree_plugin.h | ||
| tree_stall.h | ||
| tree.c | ||
| tree.h | ||
| update.c | ||