Re: [PATCH v3] srcu: Reduce blocking agressiveness of expedited grace periods further

[Marc Zyngier <maz@kernel.org>]

Hi folks,

On Fri, 01 Jul 2022 04:15:45 +0100,
Neeraj Upadhyay <quic_neeraju@quicinc.com> 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 and 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, such 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. Testing done by Marc and Zhangfei confirms that this change recovers
> most of the performance degradation in boottime; for CONFIG_HZ_250 configuration,
> boottime improves from 3m50s to 41s on Marc's setup; and from 2m40s to ~9.7s
> on Zhangfei's setup.
> 
> 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@quicinc.com>
> Tested-by: Marc Zyngier <maz@kernel.org>
> Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>

Is there any chance for this fix to make it into 5.19? The regression
is significant enough on low-end systems, and I'd rather see it
addressed.

Thanks,

	M.

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