Re: [PATCH 8/9] percpu_ref: add PERCPU_REF_INIT_* flags

Kent Overstreet
2014-09-23T21:15:15Z

On Tue, Sep 23, 2014 at 01:55:17AM -0400, Tejun Heo wrote:
> With the recent addition of percpu_ref_reinit(), percpu_ref now can be
> used as a persistent switch which can be turned on and off repeatedly
> where turning off maps to killing the ref and waiting for it to drain;
> however, there currently isn't a way to initialize a percpu_ref in its
> off (killed and drained) state, which can be inconvenient for certain
> persistent switch use cases.
>
> Similarly, percpu_ref_switch_to_atomic/percpu() allow dynamic
> selection of operation mode; however, currently a newly initialized
> percpu_ref is always in percpu mode making it impossible to avoid the
> latency overhead of switching to atomic mode.
>
> This patch adds @flags to percpu_ref_init() and implements the
> following flags.
>
> * PERCPU_REF_INIT_ATOMIC : start ref in atomic mode
> * PERCPU_REF_INIT_DEAD : start ref killed and drained
>
> These flags should be able to serve the above two use cases.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Kent Overstreet <kmo@daterainc.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>

Reviewed-by: Kent Overstreet <kmo@daterainc.com>

> ---
> block/blk-mq.c | 2 +-
> fs/aio.c | 4 ++--
> include/linux/percpu-refcount.h | 18 +++++++++++++++++-
> kernel/cgroup.c | 7 ++++---
> lib/percpu-refcount.c | 24 +++++++++++++++++++-----
> 5 files changed, 43 insertions(+), 12 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 702df07..3f6e6f5 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1777,7 +1777,7 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
> goto err_hctxs;
>
> if (percpu_ref_init(&q->mq_usage_counter, blk_mq_usage_counter_release,
> - GFP_KERNEL))
> + 0, GFP_KERNEL))
> goto err_map;
>
> setup_timer(&q->timeout, blk_mq_rq_timer, (unsigned long) q);
> diff --git a/fs/aio.c b/fs/aio.c
> index 93fbcc0f..9b6d5d6 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -666,10 +666,10 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
>
> INIT_LIST_HEAD(&ctx->active_reqs);
>
> - if (percpu_ref_init(&ctx->users, free_ioctx_users, GFP_KERNEL))
> + if (percpu_ref_init(&ctx->users, free_ioctx_users, 0, GFP_KERNEL))
> goto err;
>
> - if (percpu_ref_init(&ctx->reqs, free_ioctx_reqs, GFP_KERNEL))
> + if (percpu_ref_init(&ctx->reqs, free_ioctx_reqs, 0, GFP_KERNEL))
> goto err;
>
> ctx->cpu = alloc_percpu(struct kioctx_cpu);
> diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
> index e41ca20..5f84bf0 100644
> --- a/include/linux/percpu-refcount.h
> +++ b/include/linux/percpu-refcount.h
> @@ -61,6 +61,21 @@ enum {
> __PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD,
> };
>
> +/* @flags for percpu_ref_init() */
> +enum {
> + /*
> + * Start w/ ref == 1 in atomic mode. Can be switched to percpu
> + * operation using percpu_ref_switch_to_percpu().
> + */
> + PERCPU_REF_INIT_ATOMIC = 1 << 0,
> +
> + /*
> + * Start dead w/ ref == 0 in atomic mode. Must be revived with
> + * percpu_ref_reinit() before used. Implies INIT_ATOMIC.
> + */
> + PERCPU_REF_INIT_DEAD = 1 << 1,
> +};
> +
> struct percpu_ref {
> atomic_long_t count;
> /*
> @@ -74,7 +89,8 @@ struct percpu_ref {
> };
>
> int __must_check percpu_ref_init(struct percpu_ref *ref,
> - percpu_ref_func_t *release, gfp_t gfp);
> + percpu_ref_func_t *release, unsigned int flags,
> + gfp_t gfp);
> void percpu_ref_exit(struct percpu_ref *ref);
> void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
> percpu_ref_func_t *confirm_switch);
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 589b4d8..e2fbcc1 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1628,7 +1628,8 @@ static int cgroup_setup_root(struct cgroup_root *root, unsigned int ss_mask)
> goto out;
> root_cgrp->id = ret;
>
> - ret = percpu_ref_init(&root_cgrp->self.refcnt, css_release, GFP_KERNEL);
> + ret = percpu_ref_init(&root_cgrp->self.refcnt, css_release, 0,
> + GFP_KERNEL);
> if (ret)
> goto out;
>
> @@ -4487,7 +4488,7 @@ static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss,
>
> init_and_link_css(css, ss, cgrp);
>
> - err = percpu_ref_init(&css->refcnt, css_release, GFP_KERNEL);
> + err = percpu_ref_init(&css->refcnt, css_release, 0, GFP_KERNEL);
> if (err)
> goto err_free_css;
>
> @@ -4555,7 +4556,7 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
> goto out_unlock;
> }
>
> - ret = percpu_ref_init(&cgrp->self.refcnt, css_release, GFP_KERNEL);
> + ret = percpu_ref_init(&cgrp->self.refcnt, css_release, 0, GFP_KERNEL);
> if (ret)
> goto out_free_cgrp;
>
> diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
> index 548b19e..74ec33e 100644
> --- a/lib/percpu-refcount.c
> +++ b/lib/percpu-refcount.c
> @@ -45,27 +45,41 @@ static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
> * percpu_ref_init - initialize a percpu refcount
> * @ref: percpu_ref to initialize
> * @release: function which will be called when refcount hits 0
> + * @flags: PERCPU_REF_INIT_* flags
> * @gfp: allocation mask to use
> *
> - * Initializes the refcount in single atomic counter mode with a refcount of 1;
> - * analagous to atomic_long_set(ref, 1).
> + * Initializes @ref. If @flags is zero, @ref starts in percpu mode with a
> + * refcount of 1; analagous to atomic_long_set(ref, 1). See the
> + * definitions of PERCPU_REF_INIT_* flags for flag behaviors.
> *
> * Note that @release must not sleep - it may potentially be called from RCU
> * callback context by percpu_ref_kill().
> */
> int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
> - gfp_t gfp)
> + unsigned int flags, gfp_t gfp)
> {
> + unsigned long start_count = 0;
> +
> BUILD_BUG_ON(__PERCPU_REF_ATOMIC_DEAD &
> ~(__alignof__(unsigned long) - 1));
>
> - atomic_long_set(&ref->count, 1 + PERCPU_COUNT_BIAS);
> -
> ref->percpu_count_ptr =
> (unsigned long)alloc_percpu_gfp(unsigned long, gfp);
> if (!ref->percpu_count_ptr)
> return -ENOMEM;
>
> + if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD))
> + ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
> + else
> + start_count += PERCPU_COUNT_BIAS;
> +
> + if (flags & PERCPU_REF_INIT_DEAD)
> + ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
> + else
> + start_count++;
> +
> + atomic_long_set(&ref->count, start_count);
> +
> ref->release = release;
> return 0;
> }
> --
> 1.9.3
>