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Date:   Mon, 14 Nov 2022 13:26:53 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Uladzislau Rezki <urezki@gmail.com>, linux-kernel@vger.kernel.org,
        rcu@vger.kernel.org
Subject: Re: [PATCH v2] rcu/kfree: Do not request RCU when not needed
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On Mon, Nov 14, 2022 at 03:54:30PM -0500, Joel Fernandes wrote:
> On Mon, Nov 14, 2022 at 11:17 AM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Mon, Nov 14, 2022 at 01:20:33PM +0100, Uladzislau Rezki wrote:
> > > > On Thu, Nov 10, 2022 at 03:01:30PM +0100, Uladzislau Rezki wrote:
> > > > > > Hi,
> > > > > >
> > > > > > On Thu, Nov 10, 2022 at 8:05 AM Uladzislau Rezki <urezki@gmail.com> wrote:
> > > > > >
> > > > > > > > On ChromeOS, using this with the increased timeout, we see that we
> > > > > > > almost always
> > > > > > > > never need to initiate a new grace period. Testing also shows this frees
> > > > > > > large
> > > > > > > > amounts of unreclaimed memory, under intense kfree_rcu() pressure.
> > > > > > > >
> > > > > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > > > > > > ---
> > > > > > > > v1->v2: Same logic but use polled grace periods instead of sampling
> > > > > > > gp_seq.
> > > > > > > >
> > > > > > > >  kernel/rcu/tree.c | 8 +++++++-
> > > > > > > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > > > > > > >
> > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > > > > > index 591187b6352e..ed41243f7a49 100644
> > > > > > > > --- a/kernel/rcu/tree.c
> > > > > > > > +++ b/kernel/rcu/tree.c
> > > > > > > > @@ -2935,6 +2935,7 @@ struct kfree_rcu_cpu_work {
> > > > > > > >
> > > > > > > >  /**
> > > > > > > >   * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace
> > > > > > > period
> > > > > > > > + * @gp_snap: The GP snapshot recorded at the last scheduling of monitor
> > > > > > > work.
> > > > > > > >   * @head: List of kfree_rcu() objects not yet waiting for a grace period
> > > > > > > >   * @bkvhead: Bulk-List of kvfree_rcu() objects not yet waiting for a
> > > > > > > grace period
> > > > > > > >   * @krw_arr: Array of batches of kfree_rcu() objects waiting for a
> > > > > > > grace period
> > > > > > > > @@ -2964,6 +2965,7 @@ struct kfree_rcu_cpu {
> > > > > > > >       struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
> > > > > > > >       raw_spinlock_t lock;
> > > > > > > >       struct delayed_work monitor_work;
> > > > > > > > +     unsigned long gp_snap;
> > > > > > > >       bool initialized;
> > > > > > > >       int count;
> > > > > > > >
> > > > > > > > @@ -3167,6 +3169,7 @@ schedule_delayed_monitor_work(struct kfree_rcu_cpu
> > > > > > > *krcp)
> > > > > > > >                       mod_delayed_work(system_wq, &krcp->monitor_work,
> > > > > > > delay);
> > > > > > > >               return;
> > > > > > > >       }
> > > > > > > > +     krcp->gp_snap = get_state_synchronize_rcu();
> > > > > > > >       queue_delayed_work(system_wq, &krcp->monitor_work, delay);
> > > > > > > >  }
> > > > > > > >
> > > > > > > How do you guarantee a full grace period for objects which proceed
> > > > > > > to be placed into an input stream that is not yet detached?
> > > > > >
> > > > > >
> > > > > > Just replying from phone as I’m OOO today.
> > > > > >
> > > > > > Hmm, so you’re saying that objects can be queued after the delayed work has
> > > > > > been queued, but processed when the delayed work is run? I’m looking at
> > > > > > this code after few years so I may have missed something.
> > > > > >
> > > > > > That’s a good point and I think I missed that. I think your version did too
> > > > > > but I’ll have to double check.
> > > > > >
> > > > > > The fix then is to sample the clock for the latest object queued, not for
> > > > > > when the delayed work is queued.
> > > > > >
> > > > > The patch i sent gurantee it. Just in case see v2:
> > > >
> > > > You are right and thank you! CBs can be queued while the monitor timer is in
> > > > progress. So we need to sample unconditionally. I think my approach is still
> > > > better since I take advantage of multiple seconds (I update snapshot on every
> > > > CB queue monitor and sample in the monitor handler).
> > > >
> > > > Whereas your patch is snapshotting before queuing the regular work and when
> > > > the work is executed (This is a much shorter duration and I bet you would be
> > > > blocking in cond_synchronize..() more often).
> > > >
> > > There is a performance test that measures a taken time and memory
> > > footprint, so you can do a quick comparison. A "rcutorture" can be
> > > run with various parameters to figure out if a patch that is in question
> > > makes any difference.
> > >
> > > Usually i run it as:
> > >
> > > <snip>
> > > #! /usr/bin/env bash
> > >
> > > LOOPS=10
> > >
> > > for (( i=0; i<$LOOPS; i++ )); do
> > >         tools/testing/selftests/rcutorture/bin/kvm.sh --memory 10G --torture rcuscale --allcpus --duration 1 \
> > >         --kconfig CONFIG_NR_CPUS=64 \
> > >         --kconfig CONFIG_RCU_NOCB_CPU=y \
> > >         --kconfig CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y \
> > >         --kconfig CONFIG_RCU_LAZY=n \
> > >         --bootargs "rcuscale.kfree_rcu_test=1 rcuscale.kfree_nthreads=16 rcuscale.holdoff=20 rcuscale.kfree_loops=10000 torture.disable_onoff_at_boot" --trust-make
> > >         echo "Done $i"
> > > done
> > > <snip>
> > >
> > > just run it from your linux sandbox.
> >
> > Would it make sense to modify the "if test "$do_kvfree" = "yes" code
> > in tools/testing/selftests/rcutorture/bin/torture.sh to do something
> > like this instead of what it currently does?
> 
> Yes I think so, Were you also thinking of adding it to
> tools/testing/selftests/rcutorture/configs with a corresponding
> ".boot" file for the kfree test's boot parameters?

No, because I run torture.sh reasonably frequently, and the style of
test makes it better to run separately in that way.  In particular,
it needs lots of memory and runs for a short duration.  This is not all
that compatible with typical rcutorture runs, where lots of scenarios run
concurrently (thus limiting each VM's memory) and for long time periods.

> If it means the bots will run it more, that would be awesome :-)

I am not quite ready to have the bots run torture.sh.  Though maybe
with --do-no-scf it would be fine?

							Thanx, Paul