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Date:   Mon, 19 Jul 2021 18:03:55 -0300
From:   Marcelo Tosatti <mtosatti@redhat.com>
To:     Stefan Hajnoczi <stefanha@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Stefano Garzarella <sgarzare@redhat.com>,
        Ming Lei <ming.lei@redhat.com>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Jens Axboe <axboe@kernel.dk>, Jason Wang <jasowang@redhat.com>,
        linux-block@vger.kernel.org,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        virtualization@lists.linux-foundation.org,
        linux-pm@vger.kernel.org, Christoph Hellwig <hch@infradead.org>
Subject: Re: [RFC 1/3] cpuidle: add poll_source API
Message-ID: <20210719210355.GA46373@fuller.cnet>
References: <20210713161906.457857-1-stefanha@redhat.com>
 <20210713161906.457857-2-stefanha@redhat.com>
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In-Reply-To: <20210713161906.457857-2-stefanha@redhat.com>
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Hi Stefan,

On Tue, Jul 13, 2021 at 05:19:04PM +0100, Stefan Hajnoczi wrote:
> Introduce an API for adding cpuidle poll callbacks:
> 
>   struct poll_source_ops {
>       void (*start)(struct poll_source *src);
>       void (*stop)(struct poll_source *src);
>       void (*poll)(struct poll_source *src);
>   };
> 
>   int poll_source_register(struct poll_source *src);
>   int poll_source_unregister(struct poll_source *src);
> 
> When cpuidle enters the poll state it invokes ->start() and then invokes
> ->poll() repeatedly from the busy wait loop. Finally ->stop() is invoked
> when the busy wait loop finishes.
> 
> The ->poll() function should check for activity and cause
> TIF_NEED_RESCHED to be set in order to stop the busy wait loop.
> 
> This API is intended to be used by drivers that can cheaply poll for
> events. Participating in cpuidle polling allows them to avoid interrupt
> latencies during periods where the CPU is going to poll anyway.
> 
> Note that each poll_source is bound to a particular CPU. The API is
> mainly intended to by used by drivers that have multiple queues with irq
> affinity.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  drivers/cpuidle/Makefile      |  1 +
>  include/linux/poll_source.h   | 53 +++++++++++++++++++
>  drivers/cpuidle/poll_source.c | 99 +++++++++++++++++++++++++++++++++++
>  drivers/cpuidle/poll_state.c  |  6 +++
>  4 files changed, 159 insertions(+)
>  create mode 100644 include/linux/poll_source.h
>  create mode 100644 drivers/cpuidle/poll_source.c
> 
> diff --git a/drivers/cpuidle/Makefile b/drivers/cpuidle/Makefile
> index 26bbc5e74123..994f72d6fe95 100644
> --- a/drivers/cpuidle/Makefile
> +++ b/drivers/cpuidle/Makefile
> @@ -7,6 +7,7 @@ obj-y += cpuidle.o driver.o governor.o sysfs.o governors/
>  obj-$(CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED) += coupled.o
>  obj-$(CONFIG_DT_IDLE_STATES)		  += dt_idle_states.o
>  obj-$(CONFIG_ARCH_HAS_CPU_RELAX)	  += poll_state.o
> +obj-$(CONFIG_ARCH_HAS_CPU_RELAX)	  += poll_source.o
>  obj-$(CONFIG_HALTPOLL_CPUIDLE)		  += cpuidle-haltpoll.o
>  
>  ##################################################################################
> diff --git a/include/linux/poll_source.h b/include/linux/poll_source.h
> new file mode 100644
> index 000000000000..ccfb424e170b
> --- /dev/null
> +++ b/include/linux/poll_source.h
> @@ -0,0 +1,53 @@
> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> +/*
> + * poll_source.h - cpuidle busy waiting API
> + */
> +#ifndef __LINUX_POLLSOURCE_H__
> +#define __LINUX_POLLSOURCE_H__
> +
> +#include <linux/list.h>
> +
> +struct poll_source;
> +
> +struct poll_source_ops {
> +	void (*start)(struct poll_source *src);
> +	void (*stop)(struct poll_source *src);
> +	void (*poll)(struct poll_source *src);
> +};
> +
> +struct poll_source {
> +	const struct poll_source_ops *ops;
> +	struct list_head node;
> +	int cpu;
> +};
> +
> +/**
> + * poll_source_register - Add a poll_source for a CPU
> + */
> +#if defined(CONFIG_CPU_IDLE) && defined(CONFIG_ARCH_HAS_CPU_RELAX)
> +int poll_source_register(struct poll_source *src);
> +#else
> +static inline int poll_source_register(struct poll_source *src)
> +{
> +	return 0;
> +}
> +#endif
> +
> +/**
> + * poll_source_unregister - Remove a previously registered poll_source
> + */
> +#if defined(CONFIG_CPU_IDLE) && defined(CONFIG_ARCH_HAS_CPU_RELAX)
> +int poll_source_unregister(struct poll_source *src);
> +#else
> +static inline int poll_source_unregister(struct poll_source *src)
> +{
> +	return 0;
> +}
> +#endif
> +
> +/* Used by the cpuidle driver */
> +void poll_source_start(void);
> +void poll_source_run_once(void);
> +void poll_source_stop(void);
> +
> +#endif /* __LINUX_POLLSOURCE_H__ */
> diff --git a/drivers/cpuidle/poll_source.c b/drivers/cpuidle/poll_source.c
> new file mode 100644
> index 000000000000..46100e5a71e4
> --- /dev/null
> +++ b/drivers/cpuidle/poll_source.c
> @@ -0,0 +1,99 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * poll_source.c - cpuidle busy waiting API
> + */
> +
> +#include <linux/lockdep.h>
> +#include <linux/percpu.h>
> +#include <linux/poll_source.h>
> +
> +/* The per-cpu list of registered poll sources */
> +DEFINE_PER_CPU(struct list_head, poll_source_list);
> +
> +/* Called from idle task with TIF_POLLING_NRFLAG set and irqs enabled */
> +void poll_source_start(void)
> +{
> +	struct poll_source *src;
> +
> +	list_for_each_entry(src, this_cpu_ptr(&poll_source_list), node)
> +		src->ops->start(src);
> +}
> +
> +/* Called from idle task with TIF_POLLING_NRFLAG set and irqs enabled */
> +void poll_source_run_once(void)
> +{
> +	struct poll_source *src;
> +
> +	list_for_each_entry(src, this_cpu_ptr(&poll_source_list), node)
> +		src->ops->poll(src);
> +}
> +
> +/* Called from idle task with TIF_POLLING_NRFLAG set and irqs enabled */
> +void poll_source_stop(void)
> +{
> +	struct poll_source *src;
> +
> +	list_for_each_entry(src, this_cpu_ptr(&poll_source_list), node)
> +		src->ops->stop(src);
> +}
> +
> +static void poll_source_register_this_cpu(void *opaque)
> +{
> +	struct poll_source *src = opaque;
> +
> +	lockdep_assert_irqs_disabled();
> +
> +	list_add_tail(&src->node, this_cpu_ptr(&poll_source_list));
> +}
> +
> +int poll_source_register(struct poll_source *src)
> +{
> +	if (!list_empty(&src->node))
> +		return -EBUSY;
> +
> +	/*
> +	 * There is no race with src->cpu iterating over poll_source_list
> +	 * because smp_call_function_single() just sets TIF_NEED_RESCHED

Hum... what about 

CPU-0				CPU-1

poll_source_start		poll_source_register IPI'ing CPU-0

Perhaps a llist can be used?

>               while (!need_resched()) {
> +                     poll_source_run_once();

Probably want to use static_key's for ->start, ->stop and ->poll?