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Message-ID: <20170621155941.GA5708@htj.duckdns.org>
Date: Wed, 21 Jun 2017 11:59:41 -0400
From: Tejun Heo <tj@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>,
Jan Kara <jack@...e.com>, Nikolay Borisov <nborisov@...e.com>
Cc: jbacik@...com, linux-kernel@...r.kernel.org,
mgorman@...hsingularity.net
Subject: Re: [PATCH v3] writeback: Rework wb_[dec|inc]_stat family of
functions
Hello,
cc'ing Andrew and Jan and cc'ing whole body. The original patch
posting can be found at
https://marc.info/?l=linux-kernel&m=149802995611259&q=raw
On Wed, Jun 21, 2017 at 10:25:37AM +0300, Nikolay Borisov wrote:
> Currently the writeback statistics code uses a percpu counters to hold
> various statistics. Furthermore we have 2 families of functions - those which
> disable local irq and those which doesn't and whose names begin with
> double underscore. However, they both end up calling __add_wb_stats which in
> turn calls percpu_counter_add_batch which is already irq-safe.
>
> Exploiting this fact allows to eliminated the __wb_* functions since they don't
> add any further protection than we already have. Furthermore, refactor
> the wb_* function to call __add_wb_stat directly without the irq-disabling
> dance. This will likely result in better runtime of code which deals with
> modifying the stat counters.
>
> While at it also document why percpu_counter_add_batch is in fact preempt and
> irq-safe since at least 3 people got confused.
>
> Signed-off-by: Nikolay Borisov <nborisov@...e.com>
Acked-by: Tejun Heo <tj@...nel.org>
Andrew, this looks good to me. If you're okay with it, can you take
it through -mm? If not, I can take it through percpu although that'd
be a bit of stretch.
Thanks.
> ---
>
> Changes since v2:
> * Fixed build failure reported by kbuild test robot
> * Explicitly document that percpu_counter_add_batch is preempt/irq safe
> fs/fs-writeback.c | 8 ++++----
> include/linux/backing-dev.h | 24 ++----------------------
> lib/percpu_counter.c | 7 +++++++
> mm/page-writeback.c | 10 +++++-----
> 4 files changed, 18 insertions(+), 31 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 63ee2940775c..309364aab2a5 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -380,8 +380,8 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
> struct page *page = radix_tree_deref_slot_protected(slot,
> &mapping->tree_lock);
> if (likely(page) && PageDirty(page)) {
> - __dec_wb_stat(old_wb, WB_RECLAIMABLE);
> - __inc_wb_stat(new_wb, WB_RECLAIMABLE);
> + dec_wb_stat(old_wb, WB_RECLAIMABLE);
> + inc_wb_stat(new_wb, WB_RECLAIMABLE);
> }
> }
>
> @@ -391,8 +391,8 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
> &mapping->tree_lock);
> if (likely(page)) {
> WARN_ON_ONCE(!PageWriteback(page));
> - __dec_wb_stat(old_wb, WB_WRITEBACK);
> - __inc_wb_stat(new_wb, WB_WRITEBACK);
> + dec_wb_stat(old_wb, WB_WRITEBACK);
> + inc_wb_stat(new_wb, WB_WRITEBACK);
> }
> }
>
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index ace73f96eb1e..e9c967b86054 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -69,34 +69,14 @@ static inline void __add_wb_stat(struct bdi_writeback *wb,
> percpu_counter_add_batch(&wb->stat[item], amount, WB_STAT_BATCH);
> }
>
> -static inline void __inc_wb_stat(struct bdi_writeback *wb,
> - enum wb_stat_item item)
> -{
> - __add_wb_stat(wb, item, 1);
> -}
> -
> static inline void inc_wb_stat(struct bdi_writeback *wb, enum wb_stat_item item)
> {
> - unsigned long flags;
> -
> - local_irq_save(flags);
> - __inc_wb_stat(wb, item);
> - local_irq_restore(flags);
> -}
> -
> -static inline void __dec_wb_stat(struct bdi_writeback *wb,
> - enum wb_stat_item item)
> -{
> - __add_wb_stat(wb, item, -1);
> + __add_wb_stat(wb, item, 1);
> }
>
> static inline void dec_wb_stat(struct bdi_writeback *wb, enum wb_stat_item item)
> {
> - unsigned long flags;
> -
> - local_irq_save(flags);
> - __dec_wb_stat(wb, item);
> - local_irq_restore(flags);
> + __add_wb_stat(wb, item, -1);
> }
>
> static inline s64 wb_stat(struct bdi_writeback *wb, enum wb_stat_item item)
> diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
> index 8ee7e5ec21be..3bf4a9984f4c 100644
> --- a/lib/percpu_counter.c
> +++ b/lib/percpu_counter.c
> @@ -72,6 +72,13 @@ void percpu_counter_set(struct percpu_counter *fbc, s64 amount)
> }
> EXPORT_SYMBOL(percpu_counter_set);
>
> +/**
> + * This function is both preempt and irq safe. The former is due to explicit
> + * preemption disable. The latter is guaranteed by the fact that the slow path
> + * is explicitly protected by an irq-safe spinlock whereas the fast patch uses
> + * this_cpu_add which is irq-safe by definition. Hence there is no need muck
> + * with irq state before calling this one
> + */
> void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
> {
> s64 count;
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 143c1c25d680..b7451891959a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -601,7 +601,7 @@ static inline void __wb_writeout_inc(struct bdi_writeback *wb)
> {
> struct wb_domain *cgdom;
>
> - __inc_wb_stat(wb, WB_WRITTEN);
> + inc_wb_stat(wb, WB_WRITTEN);
> wb_domain_writeout_inc(&global_wb_domain, &wb->completions,
> wb->bdi->max_prop_frac);
>
> @@ -2437,8 +2437,8 @@ void account_page_dirtied(struct page *page, struct address_space *mapping)
> __inc_node_page_state(page, NR_FILE_DIRTY);
> __inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
> __inc_node_page_state(page, NR_DIRTIED);
> - __inc_wb_stat(wb, WB_RECLAIMABLE);
> - __inc_wb_stat(wb, WB_DIRTIED);
> + inc_wb_stat(wb, WB_RECLAIMABLE);
> + inc_wb_stat(wb, WB_DIRTIED);
> task_io_account_write(PAGE_SIZE);
> current->nr_dirtied++;
> this_cpu_inc(bdp_ratelimits);
> @@ -2745,7 +2745,7 @@ int test_clear_page_writeback(struct page *page)
> if (bdi_cap_account_writeback(bdi)) {
> struct bdi_writeback *wb = inode_to_wb(inode);
>
> - __dec_wb_stat(wb, WB_WRITEBACK);
> + dec_wb_stat(wb, WB_WRITEBACK);
> __wb_writeout_inc(wb);
> }
> }
> @@ -2791,7 +2791,7 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
> page_index(page),
> PAGECACHE_TAG_WRITEBACK);
> if (bdi_cap_account_writeback(bdi))
> - __inc_wb_stat(inode_to_wb(inode), WB_WRITEBACK);
> + inc_wb_stat(inode_to_wb(inode), WB_WRITEBACK);
>
> /*
> * We can come through here when swapping anonymous
> --
> 2.7.4
>
--
tejun
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