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From:   Doug Anderson <dianders@chromium.org>
Date:   Fri, 24 Jul 2020 10:42:55 -0700
Message-ID: <CAD=FV=WtjyYY+bmocc17S9NbRs6inkAWjj7=c9qBsVf3LtG99Q@mail.gmail.com>
Subject: Re: [PATCH] soc: qcom: rpmh-rsc: Sleep waiting for tcs slots to be free
To:     Stephen Boyd <swboyd@chromium.org>
Cc:     Andy Gross <agross@kernel.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Maulik Shah <mkshah@codeaurora.org>,
        Lina Iyer <ilina@codeaurora.org>
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Sender: linux-kernel-owner@vger.kernel.org
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Hi,

On Wed, Jul 22, 2020 at 6:01 PM Stephen Boyd <swboyd@chromium.org> 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.
>
>  rcu: INFO: rcu_preempt self-detected stall on CPU
>  rcu:     0-....: (1 GPs behind) idle=402/1/0x4000000000000002 softirq=2108/2109 fqs=4920
>   (t=21016 jiffies g=2933 q=590)
>  Task dump for CPU 0:
>  irq/11-smp2p    R  running task        0   148      2 0x00000028
>  Call trace:
>   dump_backtrace+0x0/0x154
>   show_stack+0x20/0x2c
>   sched_show_task+0xfc/0x108
>   dump_cpu_task+0x44/0x50
>   rcu_dump_cpu_stacks+0xa4/0xf8
>   rcu_sched_clock_irq+0x7dc/0xaa8
>   update_process_times+0x30/0x54
>   tick_sched_handle+0x50/0x64
>   tick_sched_timer+0x4c/0x8c
>   __hrtimer_run_queues+0x21c/0x36c
>   hrtimer_interrupt+0xf0/0x22c
>   arch_timer_handler_phys+0x40/0x50
>   handle_percpu_devid_irq+0x114/0x25c
>   __handle_domain_irq+0x84/0xc4
>   gic_handle_irq+0xd0/0x178
>   el1_irq+0xbc/0x180
>   save_return_addr+0x18/0x28
>   return_address+0x54/0x88
>   preempt_count_sub+0x40/0x88
>   _raw_spin_unlock_irqrestore+0x4c/0x6c
>   ___ratelimit+0xd0/0x128
>   rpmh_rsc_send_data+0x24c/0x378
>   __rpmh_write+0x1b0/0x208
>   rpmh_write_async+0x90/0xbc
>   rpmhpd_send_corner+0x60/0x8c
>   rpmhpd_aggregate_corner+0x8c/0x124
>   rpmhpd_set_performance_state+0x8c/0xbc
>   _genpd_set_performance_state+0xdc/0x1b8
>   dev_pm_genpd_set_performance_state+0xb8/0xf8
>   q6v5_pds_disable+0x34/0x60 [qcom_q6v5_mss]
>   qcom_msa_handover+0x38/0x44 [qcom_q6v5_mss]
>   q6v5_handover_interrupt+0x24/0x3c [qcom_q6v5]
>   handle_nested_irq+0xd0/0x138
>   qcom_smp2p_intr+0x188/0x200
>   irq_thread_fn+0x2c/0x70
>   irq_thread+0xfc/0x14c
>   kthread+0x11c/0x12c
>   ret_from_fork+0x10/0x18
>
> This busy loop naturally lends itself to using a wait queue so that each
> thread that tries to send a message will sleep waiting on the waitqueue
> and only be woken up when a free slot is available. This should make
> things more predictable too because the scheduler will be able to sleep
> tasks that are waiting on a free tcs instead of the busy loop we
> currently have today.
>
> Cc: Douglas Anderson <dianders@chromium.org>
> Cc: Maulik Shah <mkshah@codeaurora.org>
> Cc: Lina Iyer <ilina@codeaurora.org>
> Signed-off-by: Stephen Boyd <swboyd@chromium.org>
> ---
>  drivers/soc/qcom/rpmh-internal.h |   2 +
>  drivers/soc/qcom/rpmh-rsc.c      | 101 ++++++++++++-------------------
>  2 files changed, 41 insertions(+), 62 deletions(-)
>
> diff --git a/drivers/soc/qcom/rpmh-internal.h b/drivers/soc/qcom/rpmh-internal.h
> index ef60e790a750..9a325bac58fe 100644
> --- a/drivers/soc/qcom/rpmh-internal.h
> +++ b/drivers/soc/qcom/rpmh-internal.h
> @@ -8,6 +8,7 @@
>  #define __RPM_INTERNAL_H__
>
>  #include <linux/bitmap.h>
> +#include <linux/wait.h>
>  #include <soc/qcom/tcs.h>
>
>  #define TCS_TYPE_NR                    4
> @@ -118,6 +119,7 @@ struct rsc_drv {
>         struct tcs_group tcs[TCS_TYPE_NR];
>         DECLARE_BITMAP(tcs_in_use, MAX_TCS_NR);
>         spinlock_t lock;
> +       wait_queue_head_t tcs_wait;

nit: this structure has a kernel-doc comment above it describing the
elements.  Could you add yours?


>         struct rpmh_ctrlr client;
>  };
>
> diff --git a/drivers/soc/qcom/rpmh-rsc.c b/drivers/soc/qcom/rpmh-rsc.c
> index 076fd27f3081..6c758b052c95 100644
> --- a/drivers/soc/qcom/rpmh-rsc.c
> +++ b/drivers/soc/qcom/rpmh-rsc.c
> @@ -19,6 +19,7 @@
>  #include <linux/platform_device.h>
>  #include <linux/slab.h>
>  #include <linux/spinlock.h>
> +#include <linux/wait.h>
>
>  #include <soc/qcom/cmd-db.h>
>  #include <soc/qcom/tcs.h>
> @@ -444,6 +445,7 @@ static irqreturn_t tcs_tx_done(int irq, void *p)
>                  */
>                 if (!drv->tcs[ACTIVE_TCS].num_tcs)
>                         enable_tcs_irq(drv, i, false);
> +               wake_up(&drv->tcs_wait);
>                 spin_unlock(&drv->lock);

nit: I think it's slightly better to do the wake_up() after the
spin_unlock(), no?  The first thing the other task will do is to try
to grab the spinlock and we might as well give it a chance of
succeeding without looping.  I don't see any reason why we'd need to
be holding the lock while calling wake_up().


>                 if (req)
>                         rpmh_tx_done(req, err);
> @@ -562,44 +564,59 @@ static int find_free_tcs(struct tcs_group *tcs)
>         return -EBUSY;
>  }
>
> +static int claim_tcs_for_req(struct rsc_drv *drv, struct tcs_group *tcs,
> +                            const struct tcs_request *msg)

nit: I know this is a short function and kernel convention doesn't
strictly require comments in front of all functions.  However, every
other function in this file has a comment and I had a really hard time
dealing with the rpmh-rsc code before the comments.  Could you add one
for your function, even if it's short?  One thing that would be nice
to note is that the only error it returns is -EBUSY.  See below.


> +{
> +       int ret;
> +
> +       /*
> +        * The h/w does not like if we send a request to the same address,
> +        * when one is already in-flight or being processed.
> +        */
> +       ret = check_for_req_inflight(drv, tcs, msg);
> +       if (ret)
> +               return ret;
> +
> +       return find_free_tcs(tcs);
> +}
> +
>  /**
> - * tcs_write() - Store messages into a TCS right now, or return -EBUSY.
> + * rpmh_rsc_send_data() - Write / trigger active-only message.
>   * @drv: The controller.
>   * @msg: The data to be sent.
>   *
> - * Grabs a TCS for ACTIVE_ONLY transfers and writes the messages to it.
> - *
> - * If there are no free TCSes for ACTIVE_ONLY transfers or if a command for
> - * the same address is already transferring returns -EBUSY which means the
> - * client should retry shortly.
> + * NOTES:
> + * - This is only used for "ACTIVE_ONLY" since the limitations of this
> + *   function don't make sense for sleep/wake cases.
> + * - To do the transfer, we will grab a whole TCS for ourselves--we don't
> + *   try to share. If there are none available we'll wait indefinitely
> + *   for a free one.
> + * - This function will not wait for the commands to be finished, only for
> + *   data to be programmed into the RPMh. See rpmh_tx_done() which will
> + *   be called when the transfer is fully complete.
> + * - This function must be called with interrupts enabled. If the hardware
> + *   is busy doing someone else's transfer we need that transfer to fully
> + *   finish so that we can have the hardware, and to fully finish it needs
> + *   the interrupt handler to run. If the interrupts is set to run on the
> + *   active CPU this can never happen if interrupts are disabled.
>   *
> - * Return: 0 on success, -EBUSY if client should retry, or an error.
> - *         Client should have interrupts enabled for a bit before retrying.
> + * Return: 0 on success, -EINVAL on error.
>   */
> -static int tcs_write(struct rsc_drv *drv, const struct tcs_request *msg)
> +int rpmh_rsc_send_data(struct rsc_drv *drv, const struct tcs_request *msg)
>  {
>         struct tcs_group *tcs;
>         int tcs_id;
>         unsigned long flags;
> -       int ret;
>
>         tcs = get_tcs_for_msg(drv, msg);
>         if (IS_ERR(tcs))
>                 return PTR_ERR(tcs);
>
>         spin_lock_irqsave(&drv->lock, flags);
> -       /*
> -        * The h/w does not like if we send a request to the same address,
> -        * when one is already in-flight or being processed.
> -        */
> -       ret = check_for_req_inflight(drv, tcs, msg);
> -       if (ret)
> -               goto unlock;
>
> -       ret = find_free_tcs(tcs);
> -       if (ret < 0)
> -               goto unlock;
> -       tcs_id = ret;
> +       wait_event_lock_irq(drv->tcs_wait,
> +                           (tcs_id = claim_tcs_for_req(drv, tcs, msg)) >= 0,

Even though claim_tcs_for_req() only returns 0 or -EBUSY today (IOW it
never returns error codes other than -EBUSY), should we handle it?  If
we don't, claim_tcs_for_req() should be very clear that it shouldn't
return any errors other than -EBUSY.


Other than small nits this looks great to me, thanks!

-Doug