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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Mon, 14 Nov 2022 13:20:33 +0100
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Uladzislau Rezki <urezki@gmail.com>, linux-kernel@vger.kernel.org,
        paulmck@kernel.org, rcu@vger.kernel.org
Subject: Re: [PATCH v2] rcu/kfree: Do not request RCU when not needed
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> 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.

--
Uladzislau Rezki