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Subject: Re: [PATCH v2] soc: qcom: rpmh-rsc: Sleep waiting for tcs slots to be
 free
To:     Stephen Boyd <sboyd@kernel.org>, Andy Gross <agross@kernel.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>
Cc:     Stephen Boyd <swboyd@chromium.org>, linux-kernel@vger.kernel.org,
        linux-arm-msm@vger.kernel.org,
        Douglas Anderson <dianders@chromium.org>,
        Lina Iyer <ilina@codeaurora.org>
References: <20200724211711.810009-1-sboyd@kernel.org>
From:   Maulik Shah <mkshah@codeaurora.org>
Message-ID: <140f8648-2733-9823-eb84-b172b7a23be6@codeaurora.org>
Date:   Mon, 27 Jul 2020 11:09:32 +0530
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Hi,

Change looks good to me.

Reviewed-by: Maulik Shah <mkshah@codeaurora.org>

Thanks,
Maulik

On 7/25/2020 2:47 AM, Stephen Boyd wrote:
> From: Stephen Boyd <swboyd@chromium.org>
>
> 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>
> ---
>
> Changes in v2:
>   * Document tcs_wait
>   * Move wake_up() outside of the spinlock
>   * Document claim_tcs_for_req()
>
>   drivers/soc/qcom/rpmh-internal.h |   4 ++
>   drivers/soc/qcom/rpmh-rsc.c      | 115 +++++++++++++++----------------
>   2 files changed, 58 insertions(+), 61 deletions(-)
>
> diff --git a/drivers/soc/qcom/rpmh-internal.h b/drivers/soc/qcom/rpmh-internal.h
> index ef60e790a750..344ba687c13b 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
> @@ -106,6 +107,8 @@ struct rpmh_ctrlr {
>    * @lock:               Synchronize state of the controller.  If RPMH's cache
>    *                      lock will also be held, the order is: drv->lock then
>    *                      cache_lock.
> + * @tcs_wait:           Wait queue used to wait for @tcs_in_use to free up a
> + *                      slot
>    * @client:             Handle to the DRV's client.
>    */
>   struct rsc_drv {
> @@ -118,6 +121,7 @@ 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;
>   	struct rpmh_ctrlr client;
>   };
>   
> diff --git a/drivers/soc/qcom/rpmh-rsc.c b/drivers/soc/qcom/rpmh-rsc.c
> index 076fd27f3081..84a27b884af0 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>
> @@ -445,6 +446,7 @@ static irqreturn_t tcs_tx_done(int irq, void *p)
>   		if (!drv->tcs[ACTIVE_TCS].num_tcs)
>   			enable_tcs_irq(drv, i, false);
>   		spin_unlock(&drv->lock);
> +		wake_up(&drv->tcs_wait);
>   		if (req)
>   			rpmh_tx_done(req, err);
>   	}
> @@ -563,73 +565,34 @@ static int find_free_tcs(struct tcs_group *tcs)
>   }
>   
>   /**
> - * tcs_write() - Store messages into a TCS right now, or return -EBUSY.
> + * claim_tcs_for_req() - Claim a tcs in the given tcs_group; only for active.
>    * @drv: The controller.
> + * @tcs: The tcs_group used for ACTIVE_ONLY transfers.
>    * @msg: The data to be sent.
>    *
> - * Grabs a TCS for ACTIVE_ONLY transfers and writes the messages to it.
> + * Claims a tcs in the given tcs_group while making sure that no existing cmd
> + * is in flight that would conflict with the one in @msg.
>    *
> - * 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.
> + * Context: Must be called with the drv->lock held since that protects
> + * tcs_in_use.
>    *
> - * Return: 0 on success, -EBUSY if client should retry, or an error.
> - *         Client should have interrupts enabled for a bit before retrying.
> + * Return: The id of the claimed tcs or -EBUSY if a matching msg is in flight
> + * or the tcs_group is full.
>    */
> -static int tcs_write(struct rsc_drv *drv, const struct tcs_request *msg)
> +static int claim_tcs_for_req(struct rsc_drv *drv, struct tcs_group *tcs,
> +			     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;
> -
> -	tcs->req[tcs_id - tcs->offset] = msg;
> -	set_bit(tcs_id, drv->tcs_in_use);
> -	if (msg->state == RPMH_ACTIVE_ONLY_STATE && tcs->type != ACTIVE_TCS) {
> -		/*
> -		 * Clear previously programmed WAKE commands in selected
> -		 * repurposed TCS to avoid triggering them. tcs->slots will be
> -		 * cleaned from rpmh_flush() by invoking rpmh_rsc_invalidate()
> -		 */
> -		write_tcs_reg_sync(drv, RSC_DRV_CMD_ENABLE, tcs_id, 0);
> -		write_tcs_reg_sync(drv, RSC_DRV_CMD_WAIT_FOR_CMPL, tcs_id, 0);
> -		enable_tcs_irq(drv, tcs_id, true);
> -	}
> -	spin_unlock_irqrestore(&drv->lock, flags);
> -
> -	/*
> -	 * These two can be done after the lock is released because:
> -	 * - We marked "tcs_in_use" under lock.
> -	 * - Once "tcs_in_use" has been marked nobody else could be writing
> -	 *   to these registers until the interrupt goes off.
> -	 * - The interrupt can't go off until we trigger w/ the last line
> -	 *   of __tcs_set_trigger() below.
> -	 */
> -	__tcs_buffer_write(drv, tcs_id, 0, msg);
> -	__tcs_set_trigger(drv, tcs_id, true);
> +		return ret;
>   
> -	return 0;
> -unlock:
> -	spin_unlock_irqrestore(&drv->lock, flags);
> -	return ret;
> +	return find_free_tcs(tcs);
>   }
>   
>   /**
> @@ -656,18 +619,47 @@ 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)
>   {
> -	int ret;
> +	struct tcs_group *tcs;
> +	int tcs_id;
> +	unsigned long flags;
>   
> -	do {
> -		ret = tcs_write(drv, msg);
> -		if (ret == -EBUSY) {
> -			pr_info_ratelimited("TCS Busy, retrying RPMH message send: addr=%#x\n",
> -					    msg->cmds[0].addr);
> -			udelay(10);
> -		}
> -	} while (ret == -EBUSY);
> +	tcs = get_tcs_for_msg(drv, msg);
> +	if (IS_ERR(tcs))
> +		return PTR_ERR(tcs);
>   
> -	return ret;
> +	spin_lock_irqsave(&drv->lock, flags);
> +
> +	/* Wait forever for a free tcs. It better be there eventually! */
> +	wait_event_lock_irq(drv->tcs_wait,
> +			    (tcs_id = claim_tcs_for_req(drv, tcs, msg)) >= 0,
> +			    drv->lock);
> +
> +	tcs->req[tcs_id - tcs->offset] = msg;
> +	set_bit(tcs_id, drv->tcs_in_use);
> +	if (msg->state == RPMH_ACTIVE_ONLY_STATE && tcs->type != ACTIVE_TCS) {
> +		/*
> +		 * Clear previously programmed WAKE commands in selected
> +		 * repurposed TCS to avoid triggering them. tcs->slots will be
> +		 * cleaned from rpmh_flush() by invoking rpmh_rsc_invalidate()
> +		 */
> +		write_tcs_reg_sync(drv, RSC_DRV_CMD_ENABLE, tcs_id, 0);
> +		write_tcs_reg_sync(drv, RSC_DRV_CMD_WAIT_FOR_CMPL, tcs_id, 0);
> +		enable_tcs_irq(drv, tcs_id, true);
> +	}
> +	spin_unlock_irqrestore(&drv->lock, flags);
> +
> +	/*
> +	 * These two can be done after the lock is released because:
> +	 * - We marked "tcs_in_use" under lock.
> +	 * - Once "tcs_in_use" has been marked nobody else could be writing
> +	 *   to these registers until the interrupt goes off.
> +	 * - The interrupt can't go off until we trigger w/ the last line
> +	 *   of __tcs_set_trigger() below.
> +	 */
> +	__tcs_buffer_write(drv, tcs_id, 0, msg);
> +	__tcs_set_trigger(drv, tcs_id, true);
> +
> +	return 0;
>   }
>   
>   /**
> @@ -975,6 +967,7 @@ static int rpmh_rsc_probe(struct platform_device *pdev)
>   		return ret;
>   
>   	spin_lock_init(&drv->lock);
> +	init_waitqueue_head(&drv->tcs_wait);
>   	bitmap_zero(drv->tcs_in_use, MAX_TCS_NR);
>   
>   	irq = platform_get_irq(pdev, drv->id);

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