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