Re: [PATCH v7 2/5] block: add task-context bio completion infrastructure

[Jan Kara]

On Thu 30-07-26 02:57:52, Tal Zussman wrote:
> Some bio completion handlers need to run from preemptible task context,
> but bio_endio() may be called from IRQ context (e.g., buffer_head
> writeback). Callers need a way to ensure their callback eventually runs
> from a sleepable context. Add infrastructure for that, in two forms:
> 
>   1. BIO_COMPLETE_IN_TASK, a bio flag the submitter sets when it knows
>      in advance that its callback needs task context (e.g., dropbehind
>      writeback). bio_endio() sees the flag and offloads completion to a
>      worker automatically.
> 
>   2. bio_complete_in_task(), a helper that completion callbacks can
>      invoke from within bi_end_io() when the deferral decision is
>      dynamic (e.g., fserror reporting).
> 
> Both share a per-CPU list drained by a work item on a WQ_PERCPU
> workqueue. Producers push the bio onto the local CPU's list and schedule
> the work item, which then dispatches each bio's bi_end_io() from task
> context.
> 
> Both methods are gated on bio_in_atomic(), which returns true in any
> context where a sleeping bi_end_io() is unsafe, including
> non-preemptible task context.
> 
> Two CPU hotplug callbacks are used to drain remaining bios from the
> departing CPU's batch, while maintaining the per-CPU behavior. The
> CPUHP_AP_ONLINE_DYN callback disables the per-CPU work item while the
> CPU is still online, preventing it from running on an unbound worker
> later. CPUHP_BP_PREPARE_DYN then drains any bios added between disabling
> the work item and CPU offline.
> 
> Link: https://lore.kernel.org/all/20260409160243.1008358-1-hch@xxxxxx/
> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>

Looks good! Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  block/bio.c               | 132 +++++++++++++++++++++++++++++++++++++++++++++-
>  include/linux/bio.h       |  24 +++++++++
>  include/linux/blk_types.h |   1 +
>  3 files changed, 156 insertions(+), 1 deletion(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 6a2f6fc3413e..a9610950325b 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1741,6 +1741,61 @@ void bio_check_pages_dirty(struct bio *bio)
>  	schedule_work(&bio_dirty_work);
>  }
>  
> +/*
> + * Infrastructure for deferring bio completions to task-context via a per-CPU
> + * workqueue. Triggered either by the BIO_COMPLETE_IN_TASK bio flag (static
> + * decision at submit time) or by calling bio_complete_in_task() from
> + * bi_end_io() (dynamic decision at completion time).
> + */
> +
> +struct bio_complete_batch {
> +	struct bio_list list;
> +	struct work_struct work;
> +	int cpu;
> +};
> +
> +static DEFINE_PER_CPU(struct bio_complete_batch, bio_complete_batch);
> +static struct workqueue_struct *bio_complete_wq;
> +
> +static void bio_complete_work_fn(struct work_struct *w)
> +{
> +	struct bio_complete_batch *batch =
> +		container_of(w, struct bio_complete_batch, work);
> +
> +	while (1) {
> +		struct bio_list list;
> +		struct bio *bio;
> +
> +		local_irq_disable();
> +		list = batch->list;
> +		bio_list_init(&batch->list);
> +		local_irq_enable();
> +
> +		if (bio_list_empty(&list))
> +			break;
> +
> +		while ((bio = bio_list_pop(&list)))
> +			bio->bi_end_io(bio);
> +	}
> +}
> +
> +void __bio_complete_in_task(struct bio *bio)
> +{
> +	struct bio_complete_batch *batch;
> +	unsigned long flags;
> +	bool was_empty;
> +
> +	local_irq_save(flags);
> +	batch = this_cpu_ptr(&bio_complete_batch);
> +	was_empty = bio_list_empty(&batch->list);
> +	bio_list_add(&batch->list, bio);
> +	local_irq_restore(flags);
> +
> +	if (was_empty)
> +		queue_work_on(batch->cpu, bio_complete_wq, &batch->work);
> +}
> +EXPORT_SYMBOL_GPL(__bio_complete_in_task);
> +
>  static inline bool bio_remaining_done(struct bio *bio)
>  {
>  	/*
> @@ -1815,7 +1870,9 @@ void bio_endio(struct bio *bio)
>  	}
>  #endif
>  
> -	if (bio->bi_end_io)
> +	if (bio_flagged(bio, BIO_COMPLETE_IN_TASK) && bio_in_atomic())
> +		__bio_complete_in_task(bio);
> +	else if (bio->bi_end_io)
>  		bio->bi_end_io(bio);
>  }
>  EXPORT_SYMBOL(bio_endio);
> @@ -2001,6 +2058,55 @@ int bioset_init(struct bio_set *bs,
>  }
>  EXPORT_SYMBOL(bioset_init);
>  
> +static int bio_complete_batch_cpu_online(unsigned int cpu)
> +{
> +	struct bio_complete_batch *batch = &per_cpu(bio_complete_batch, cpu);
> +
> +	enable_work(&batch->work);
> +	if (!bio_list_empty(&batch->list))
> +		queue_work_on(cpu, bio_complete_wq, &batch->work);
> +	return 0;
> +}
> +
> +/*
> + * Disable this CPU's work item so that it cannot run on an unbound worker
> + * after the CPU is offlined.
> + */
> +static int bio_complete_batch_cpu_down_prep(unsigned int cpu)
> +{
> +	disable_work_sync(&per_cpu(bio_complete_batch, cpu).work);
> +	return 0;
> +}
> +
> +/*
> + * Drain a dead CPU's deferred bio completions. The CPU is dead and the worker
> + * is canceled so no locking is needed.
> + */
> +static int bio_complete_batch_cpu_dead(unsigned int cpu)
> +{
> +	struct bio_complete_batch *batch =
> +		per_cpu_ptr(&bio_complete_batch, cpu);
> +	struct bio *bio;
> +
> +	while ((bio = bio_list_pop(&batch->list)))
> +		bio->bi_end_io(bio);
> +
> +	return 0;
> +}
> +
> +static void __init bio_complete_batch_init(int cpu)
> +{
> +	struct bio_complete_batch *batch =
> +		per_cpu_ptr(&bio_complete_batch, cpu);
> +
> +	bio_list_init(&batch->list);
> +	INIT_WORK(&batch->work, bio_complete_work_fn);
> +	batch->cpu = cpu;
> +
> +	if (!cpu_online(cpu))
> +		disable_work_sync(&batch->work);
> +}
> +
>  static int __init init_bio(void)
>  {
>  	int i;
> @@ -2015,6 +2121,30 @@ static int __init init_bio(void)
>  				SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);
>  	}
>  
> +	for_each_possible_cpu(i)
> +		bio_complete_batch_init(i);
> +
> +	bio_complete_wq = alloc_workqueue("bio_complete",
> +					   WQ_MEM_RECLAIM | WQ_PERCPU, 0);
> +	if (!bio_complete_wq)
> +		panic("bio: can't allocate bio_complete workqueue\n");
> +
> +	/*
> +	 * bio task-context completion draining on hot-unplugged CPUs:
> +	 *
> +	 *   1. Stop the per-CPU work item while the CPU is still online, so
> +	 *      that it cannot run on an unbound worker later.
> +	 *   2. Drain leftover bios added between worker disabling and CPU
> +	 *      offlining.
> +	 */
> +	cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
> +				  "block/bio:complete:online",
> +				  bio_complete_batch_cpu_online,
> +				  bio_complete_batch_cpu_down_prep);
> +	cpuhp_setup_state_nocalls(CPUHP_BP_PREPARE_DYN,
> +				  "block/bio:complete:dead",
> +				  NULL, bio_complete_batch_cpu_dead);
> +
>  	cpuhp_setup_state_multi(CPUHP_BIO_DEAD, "block/bio:dead", NULL,
>  					bio_cpu_dead);
>  
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 62a983758e09..a8091a3e9d87 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -383,6 +383,30 @@ static inline bool bio_in_atomic(void)
>  	return !preemptible();
>  }
>  
> +void __bio_complete_in_task(struct bio *bio);
> +
> +/**
> + * bio_complete_in_task - ensure a bio is completed in preemptible task context
> + * @bio: bio to complete
> + *
> + * If called from non-task context, offload the bio completion to a worker
> + * thread and return %true. Else return %false and do nothing.
> + *
> + * Uses BIO_COMPLETE_IN_TASK as a sentinel: if set, the bio was already
> + * deferred and we are running in the worker — return %false so the
> + * callback proceeds instead of re-deferring.
> + */
> +static inline bool bio_complete_in_task(struct bio *bio)
> +{
> +	if (bio_flagged(bio, BIO_COMPLETE_IN_TASK))
> +		return false;
> +	if (!bio_in_atomic())
> +		return false;
> +	bio_set_flag(bio, BIO_COMPLETE_IN_TASK);
> +	__bio_complete_in_task(bio);
> +	return true;
> +}
> +
>  extern void bio_endio(struct bio *);
>  
>  /**
> diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
> index 8808ee76e73c..d49d97a050d0 100644
> --- a/include/linux/blk_types.h
> +++ b/include/linux/blk_types.h
> @@ -322,6 +322,7 @@ enum {
>  	BIO_REMAPPED,
>  	BIO_ZONE_WRITE_PLUGGING, /* bio handled through zone write plugging */
>  	BIO_EMULATES_ZONE_APPEND, /* bio emulates a zone append operation */
> +	BIO_COMPLETE_IN_TASK, /* complete bi_end_io() in task context */
>  	BIO_FLAG_LAST
>  };
>  
> 
> -- 
> 2.39.5
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR