Re: [PATCH v3] writeback: Rework wb_[dec|inc]_stat family of functions

[Jan Kara]

On Wed 21-06-17 11:59:41, Tejun Heo wrote:
> 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@xxxxxxxx>
> 
>   Acked-by: Tejun Heo <tj@xxxxxxxxxx>
> 
> 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.

The patch looks good to me as well. You can add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> 
> 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
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR