linux-kernel - Re: [PATCH] soc: qcom: rpmh-rsc: Sleep waiting for tcs slots to be free

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Message-ID: <159553296815.3847286.2798719474122080066@swboyd.mtv.corp.google.com>
Date:   Thu, 23 Jul 2020 12:36:08 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Lina Iyer <ilina@...eaurora.org>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>,
        Maulik Shah <mkshah@...eaurora.org>
Subject: Re: [PATCH] soc: qcom: rpmh-rsc: Sleep waiting for tcs slots to be free

Quoting Lina Iyer (2020-07-23 10:42:54)
> On Wed, Jul 22 2020 at 19:01 -0600, Stephen Boyd wrote:
> >The busy loop in rpmh_rsc_send_data() is written with the assumption
> >that the udelay will be preempted by the tcs_tx_done() irq handler when
> >the TCS slots are all full. This doesn't hold true when the calling
> >thread is an irqthread and the tcs_tx_done() irq is also an irqthread.
> >That's because kernel irqthreads are SCHED_FIFO and thus need to
> >voluntarily give up priority by calling into the scheduler so that other
> >threads can run.
> >
> >I see RCU stalls when I boot with irqthreads on the kernel commandline
> >because the modem remoteproc driver is trying to send an rpmh async
> >message from an irqthread that needs to give up the CPU for the rpmh
> >irqthread to run and clear out tcs slots.
> >
> Would this be not better, if we we use a threaded IRQ handler or offload
> tx_done to another waitqueue instead of handling it in IRQ handler?
> 

Are you asking if jitter is reduced when the rpmh irq is made into a
threaded irq? I haven't done any benchmarking to see if it improves
things.