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

[Marc Zyngier]

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).

Anyway:

Tested-by: Marc Zyngier <maz@xxxxxxxxxx>

	M.

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