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 <nbori...@suse.com> Acked-by: Tejun Heo <t...@kernel.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
