linux-kernel - Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking readers from consuming CPU) cause qemu boot slow

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Message-ID: <7f6d9360-9254-88d6-fb34-13f248d2e542@redhat.com>
Date:   Wed, 15 Jun 2022 12:50:14 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        "zhangfei.gao@...mail.com" <zhangfei.gao@...mail.com>,
        paulmck@...nel.org, zhangfei <zhangfei.gao@...aro.org>
Cc:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Matthew Wilcox <willy@...radead.org>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>,
        Auger Eric <eric.auger@...hat.com>,
        "chenxiang (M)" <chenxiang66@...ilicon.com>
Subject: Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking
 readers from consuming CPU) cause qemu boot slow

On 6/15/22 12:40, Neeraj Upadhyay wrote:
> 
> This is useful data, thanks! Did you get chance to check between 100 and 
> 1000, to narrow down further, from which point (does need to be exact 
> value) between 100 and 1000,  you start seeing degradation at, for ex. 
> 250, 500 , ...?
> 
> Is it also possible to try experiment 10 and 11 with below diff.
> What I have done in below diff is, call srcu_get_delay() only once
> in try_check_zero() (and not for every loop iteration); also
> retry with a different delay for the extra iteration which is done
> when srcu_get_delay(ssp) returns 0.
> 
> Once we have this data, can you also try by changing 
> SRCU_RETRY_CHECK_LONG_DELAY   to 100, on top of below diff.
> 
> #define SRCU_RETRY_CHECK_LONG_DELAY  100

Is there any data that you would like me to gather from the KVM side, 
for example with respect to how much it takes to do synchronize_srcu() 
on an unpatched kernel, or the duration of the read-sides?

Thanks,

Paolo