On Mon, 2018-07-09 at 15:08 -0700, Paul E. McKenney wrote: > > And the earlier patch was against my -rcu tree, which won't be all that > helpful for v4.15. Please see below for a lightly tested backport to v4.15. > > It should apply to all the releases of interest. If other backports > are needed, please remind me of my woodhouse.v4.15.2018.07.09a tag. > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 6361b81827a8f93f582124da385258fc04a38a7f > Author: Paul E. McKenney <paul...@linux.vnet.ibm.com> > Date: Mon Jul 9 13:47:30 2018 -0700 > > rcu: Make need_resched() respond to urgent RCU-QS needs > > The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent > need for an RCU quiescent state from the force-quiescent-state processing > within the grace-period kthread to context switches and to cond_resched(). > Unfortunately, such urgent needs are not communicated to need_resched(), > which is sometimes used to decide when to invoke cond_resched(), for > but one example, within the KVM vcpu_run() function. As of v4.15, this > can result in synchronize_sched() being delayed by up to ten seconds, > which can be problematic, to say nothing of annoying. > > This commit therefore checks rcu_dynticks.rcu_urgent_qs from within > rcu_check_callbacks(), which is invoked from the scheduling-clock > interrupt handler. If the current task is not an idle task and is > not executing in usermode, a context switch is forced, and either way, > the rcu_dynticks.rcu_urgent_qs variable is set to false. If the current > task is an idle task, then RCU's dyntick-idle code will detect the > quiescent state, so no further action is required. Similarly, if the > task is executing in usermode, other code in rcu_check_callbacks() and > its called functions will report the corresponding quiescent state. > > Reported-by: David Woodhouse <dw...@infradead.org> > Suggested-by: Peter Zijlstra <pet...@infradead.org> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> > [ paulmck: Backported to v4.15. Probably applies elsewhere. ]
Hm, this doesn't appear to work. I'm still seeing latencies of 4-5
seconds in my testing. In fact, even our old workaround of adding
rcu_all_qs() into vcpu_enter_guest() didn't properly fix it AFAICT.
I'm just creating a VM with lots of CPUs, then attaching new devices to
it to cause the VMM to open more file descriptors, until it hits a
power of two and invokes expand_fdtable().
expand_fdtable (512) sync took 10472394964 cycles (3500000 µs).
expand_fdtable (512) sync took 15298908072 cycles (5100000 µs).
--- a/fs/file.c
+++ b/fs/file.c
@@ -162,8 +162,16 @@ static int expand_fdtable(struct files_struct *files,
unsigned int nr)
/* make sure all __fd_install() have seen resize_in_progress
* or have finished their rcu_read_lock_sched() section.
*/
- if (atomic_read(&files->count) > 1)
+ if (atomic_read(&files->count) > 1) {
+ unsigned long sync_start, sync_end;
+ unsigned long j_start, j_end;
+ j_start = jiffies;
+ sync_start = get_cycles();
synchronize_sched();
+ sync_end = get_cycles();
+ j_end = jiffies;
+ printk("expand_fdtable (%d) sync took %ld cycles (%ld µs).\n",
nr, sync_end - sync_start, jiffies_to_usecs(j_end - j_start));
+ }
spin_lock(&files->file_lock);
if (!new_fdt)
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