Re: [PATCH] srcu: Reduce blocking agressiveness of expedited grace periods further

[Neeraj Upadhyay]

On 6/28/2022 3:01 PM, Marc Zyngier wrote:
> On Tue, 28 Jun 2022 10:17:24 +0100,
> Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx> wrote:
> > On 6/28/2022 2:32 PM, Marc Zyngier wrote:
> > > On Mon, 27 Jun 2022 13:37:06 +0100,
> > > Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx> wrote:
> > > > 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.
> > > >
> > > > 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:
> > > >
> > > > +──────────────────────────+────────────────+
> > > > | SRCU_MAX_NODELAY_PHASE   | Boot time (s)  |
> > > > +──────────────────────────+────────────────+
> > > > | 100                      | 30.053         |
> > > > | 150                      | 25.151         |
> > > > | 200                      | 20.704         |
> > > > | 250                      | 15.748         |
> > > > | 500                      | 11.401         |
> > > > | 1000                     | 11.443         |
> > > > | 10000                    | 11.258         |
> > > > | 1000000                  | 11.154         |
> > > > +──────────────────────────+────────────────+
> > > >
> > > > 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.
> > > >
> > > > So, this change scales per-grace-period phase number of non-sleeping
> > > > polls, soiuch that, non-sleeping polls are done for one jiffy. In addition
> > > > to this, srcu_get_delay() call in srcu_gp_end(), which is used to calculate
> > > > the delay used for scheduling callbacks, is replaced with the check for
> > > > expedited grace period. This is done, to schedule cbs for completed expedited
> > > > grace periods immediately, which results in improved boot time seen in
> > > > experiments.
> > > >
> > > > 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.
> > > >
> > > > Signed-off-by: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> > >
> > > 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=250).
> > >
> > > 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).
> >
> > Do you see any improvement by using "srcutree.srcu_max_nodelay=1000"
> > 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.

Cool, thanks!

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

It need to be tuned :) The patch actually adds 2 jiffies (vs the one 
jiffy mentioned in the commit log) of non-sleep per phase delay. Each 
phase iteration uses a delay of 10 us. So, for CONFIG_HZ_250 its around 
800 iterations.


Thanks
Neeraj

> Thanks,
>
> 	M.