On Thu, May 09, 2024 at 05:16:57AM -0300, Leonardo Bras wrote: > On Wed, May 08, 2024 at 08:32:40PM -0700, Paul E. McKenney wrote: > > On Wed, May 08, 2024 at 07:01:29AM -0700, Sean Christopherson wrote: > > > On Wed, May 08, 2024, Leonardo Bras wrote: > > > > Something just hit me, and maybe I need to propose something more generic. > > > > > > Yes. This is what I was trying to get across with my complaints about keying off > > > of the last VM-Exit time. It's effectively a broad stroke "this task will likely > > > be quiescent soon" and so the core concept/functionality belongs in common code, > > > not KVM. > > > > OK, we could do something like the following wholly within RCU, namely > > to make rcu_pending() refrain from invoking rcu_core() until the grace > > period is at least the specified age, defaulting to zero (and to the > > current behavior). > > > > Perhaps something like the patch shown below. > > That's exactly what I was thinking :) > > > > > Thoughts? > > Some suggestions below: > > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > commit abc7cd2facdebf85aa075c567321589862f88542 > > Author: Paul E. McKenney <paulmck@xxxxxxxxxx> > > Date: Wed May 8 20:11:58 2024 -0700 > > > > rcu: Add rcutree.nocb_patience_delay to reduce nohz_full OS jitter > > > > 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.nocb_patience_delay kernel boot > > parameter that specifies the grace period age (in milliseconds) > > before which RCU will refrain from awakening the rcuc kthread. > > Preliminary experiementation suggests a value of 1000, that is, > > one second. Increasing rcutree.nocb_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. > > > > Link: https://lore.kernel.org/all/20240328171949.743211-1-leobras@xxxxxxxxxx/ > > > > Reported-by: Leonardo Bras <leobras@xxxxxxxxxx> > > Suggested-by: Leonardo Bras <leobras@xxxxxxxxxx> > > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > > index 0a3b0fd1910e6..42383986e692b 100644 > > --- a/Documentation/admin-guide/kernel-parameters.txt > > +++ b/Documentation/admin-guide/kernel-parameters.txt > > @@ -4981,6 +4981,13 @@ > > the ->nocb_bypass queue. The definition of "too > > many" is supplied by this kernel boot parameter. > > > > + rcutree.nocb_patience_delay= [KNL] > > + On callback-offloaded (rcu_nocbs) CPUs, avoid > > + disturbing RCU unless the grace period has > > + reached the specified age in milliseconds. > > + Defaults to zero. Large values will be capped > > + at five seconds. > > + > > rcutree.qhimark= [KNL] > > Set threshold of queued RCU callbacks beyond which > > batch limiting is disabled. > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index 7560e204198bb..6e4b8b43855a0 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -176,6 +176,8 @@ static int gp_init_delay; > > module_param(gp_init_delay, int, 0444); > > static int gp_cleanup_delay; > > module_param(gp_cleanup_delay, int, 0444); > > +static int nocb_patience_delay; > > +module_param(nocb_patience_delay, int, 0444); > > > > // Add delay to rcu_read_unlock() for strict grace periods. > > static int rcu_unlock_delay; > > @@ -4334,6 +4336,8 @@ EXPORT_SYMBOL_GPL(cond_synchronize_rcu_full); > > static int rcu_pending(int user) > > { > > bool gp_in_progress; > > + unsigned long j = jiffies; > > I think this is probably taken care by the compiler, but just in case I would move the > j = jiffies; > closer to it's use, in order to avoid reading 'jiffies' if rcu_pending > exits before the nohz_full testing. Good point! I just removed j and used jiffies directly. > > + unsigned int patience = msecs_to_jiffies(nocb_patience_delay); > > What do you think on processsing the new parameter in boot, and saving it > in terms of jiffies already? > > It would make it unnecessary to convert ms -> jiffies every time we run > rcu_pending. > > (OOO will probably remove the extra division, but may cause less impact in > some arch) This isn't exactly a fastpath, but it is easy enough to do the conversion in rcu_bootup_announce_oddness() and place it into another variable (for the benefit of those using drgn or going through crash dumps). > > struct rcu_data *rdp = this_cpu_ptr(&rcu_data); > > struct rcu_node *rnp = rdp->mynode; > > > > @@ -4347,11 +4351,13 @@ static int rcu_pending(int user) > > return 1; > > > > /* Is this a nohz_full CPU in userspace or idle? (Ignore RCU if so.) */ > > - if ((user || rcu_is_cpu_rrupt_from_idle()) && rcu_nohz_full_cpu()) > > + gp_in_progress = rcu_gp_in_progress(); > > + if ((user || rcu_is_cpu_rrupt_from_idle() || > > + (gp_in_progress && time_before(j + patience, rcu_state.gp_start))) && > > I think you meant: > time_before(j, rcu_state.gp_start + patience) > > or else this always fails, as we can never have now to happen before a > previously started gp, right? > > Also, as per rcu_nohz_full_cpu() we probably need it to be read with > READ_ONCE(): > > time_before(j, READ_ONCE(rcu_state.gp_start) + patience) Good catch on both counts, fixed! > > + rcu_nohz_full_cpu()) > > return 0; > > > > /* Is the RCU core waiting for a quiescent state from this CPU? */ > > - gp_in_progress = rcu_gp_in_progress(); > > if (rdp->core_needs_qs && !rdp->cpu_no_qs.b.norm && gp_in_progress) > > return 1; > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index 340bbefe5f652..174333d0e9507 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -93,6 +93,15 @@ static void __init rcu_bootup_announce_oddness(void) > > pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_init_delay); > > if (gp_cleanup_delay) > > pr_info("\tRCU debug GP cleanup slowdown %d jiffies.\n", gp_cleanup_delay); > > + if (nocb_patience_delay < 0) { > > + pr_info("\tRCU NOCB CPU patience negative (%d), resetting to zero.\n", nocb_patience_delay); > > + nocb_patience_delay = 0; > > + } else if (nocb_patience_delay > 5 * MSEC_PER_SEC) { > > + pr_info("\tRCU NOCB CPU patience too large (%d), resetting to %ld.\n", nocb_patience_delay, 5 * MSEC_PER_SEC); > > + nocb_patience_delay = 5 * MSEC_PER_SEC; > > + } else if (nocb_patience_delay) { > > Here you suggest that we don't print if 'nocb_patience_delay == 0', > as it's the default behavior, right? Exactly, in keeping with the function name rcu_bootup_announce_oddness(). This approach allows easy spotting of deviations from default settings, which can be very helpful when debugging. > I think printing on 0 could be useful to check if the feature exists, even > though we are zeroing it, but this will probably add unnecessary verbosity. It could be quite useful to people learning the RCU implementation, and I encourage those people to remove all those "if" statements from rcu_bootup_announce_oddness() in order to get the full story. > > + pr_info("\tRCU NOCB CPU patience set to %d milliseconds.\n", nocb_patience_delay); > > + } > > Here I suppose something like this can take care of not needing to convert > ms -> jiffies every rcu_pending(): > > + nocb_patience_delay = msecs_to_jiffies(nocb_patience_delay); Agreed, but I used a separate variable to help people looking at crash dumps or using drgn. And thank you for your review and comments! Applying these changes with attribution. Thanx, Paul > > if (!use_softirq) > > pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.\n"); > > if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG)) > > > > > Thanks! > Leo >
