Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 28 Jun 2022 10:31:54 +0100
Message-ID: <874k052kxh.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc:     <paulmck@kernel.org>, <frederic@kernel.org>,
        <josh@joshtriplett.org>, <rostedt@goodmis.org>,
        <mathieu.desnoyers@efficios.com>, <jiangshanlai@gmail.com>,
        <joel@joelfernandes.org>, <linux-kernel@vger.kernel.org>,
        <zhangfei.gao@foxmail.com>, <boqun.feng@gmail.com>,
        <urezki@gmail.com>, <shameerali.kolothum.thodi@huawei.com>,
        <pbonzini@redhat.com>, <mtosatti@redhat.com>,
        <eric.auger@redhat.com>, <chenxiang66@hisilicon.com>
Subject: Re: [PATCH] srcu: Reduce blocking agressiveness of expedited grace periods further
In-Reply-To: <dc24db89-05ae-c113-6728-de797e041123@quicinc.com>
References: <20220627123706.20187-1-quic_neeraju@quicinc.com>
        <875ykl2mb2.wl-maz@kernel.org>
        <dc24db89-05ae-c113-6728-de797e041123@quicinc.com>
User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue)
 FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/27.1
 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO)
MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue")
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Tue, 28 Jun 2022 10:17:24 +0100,
Neeraj Upadhyay <quic_neeraju@quicinc.com> wrote:
>=20
>=20
>=20
> On 6/28/2022 2:32 PM, Marc Zyngier wrote:
> > On Mon, 27 Jun 2022 13:37:06 +0100,
> > Neeraj Upadhyay <quic_neeraju@quicinc.com> wrote:
> >>=20
> >> Commit 640a7d37c3f4 ("srcu: Block less aggressively for expedited
> >> grace periods") highlights a problem where aggressively blocking
> >> SRCU expedited grace periods, as was introduced in commit
> >> 282d8998e997 ("srcu: Prevent expedited GPs and blocking readers
> >> from consuming CPU"), introduces ~2 minutes delay to the overall
> >> ~3.5 minutes boot time, when starting VMs with "-bios QEMU_EFI.fd"
> >> cmdline on qemu, which results in very high rate of memslots
> >> add/remove, which causes > ~6000 synchronize_srcu() calls for
> >> kvm->srcu SRCU instance.
> >>=20
> >> Below table captures the experiments done by Zhangfei Gao, Shameer,
> >> to measure the boottime impact with various values of non-sleeping
> >> per phase counts, with HZ_250 and preemption enabled:
> >>=20
> >> +=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=
=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=
=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=
=94=80=E2=94=80+=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=
=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=
=80=E2=94=80+
> >> | SRCU_MAX_NODELAY_PHASE   | Boot time (s)  |
> >> +=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=
=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=
=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=
=94=80=E2=94=80+=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=
=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=
=80=E2=94=80+
> >> | 100                      | 30.053         |
> >> | 150                      | 25.151         |
> >> | 200                      | 20.704         |
> >> | 250                      | 15.748         |
> >> | 500                      | 11.401         |
> >> | 1000                     | 11.443         |
> >> | 10000                    | 11.258         |
> >> | 1000000                  | 11.154         |
> >> +=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=
=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=
=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=
=94=80=E2=94=80+=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=
=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=
=80=E2=94=80+
> >>=20
> >> Analysis on the experiment results showed improved boot time
> >> with non blocking delays close to one jiffy duration. This
> >> was also seen when number of per-phase iterations were scaled
> >> to one jiffy.
> >>=20
> >> So, this change scales per-grace-period phase number of non-sleeping
> >> polls, soiuch that, non-sleeping polls are done for one jiffy. In addi=
tion
> >> to this, srcu_get_delay() call in srcu_gp_end(), which is used to calc=
ulate
> >> the delay used for scheduling callbacks, is replaced with the check for
> >> expedited grace period. This is done, to schedule cbs for completed ex=
pedited
> >> grace periods immediately, which results in improved boot time seen in
> >> experiments.
> >>=20
> >> In addition to the changes to default per phase delays, this change
> >> adds 3 new kernel parameters - srcutree.srcu_max_nodelay,
> >> srcutree.srcu_max_nodelay_phase, srcutree.srcu_retry_check_delay.
> >> This allows users to configure the srcu grace period scanning delays,
> >> depending on their system configuration requirements.
> >>=20
> >> Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> >=20
> > I've given this a go on one of my test platforms (the one I noticed
> > the issue on the first place), and found that the initial part of the
> > EFI boot under KVM (pointlessly wiping the emulated flash) went down
> > to 1m7s from 3m50s (HZ=3D250).
> >=20
> > Clearly a massive improvement, but still a far cry from the original
> > ~40s (yes, this box is utter crap -- which is why I use it).
>=20
> Do you see any improvement by using "srcutree.srcu_max_nodelay=3D1000"
> bootarg, on top of this patch?

Yup, this brings it back to 43s on a quick test run, which is close
enough to what I had before.

How does a random user come up with such a value though?

Thanks,

	M.

--=20
Without deviation from the norm, progress is not possible.