Messages in this thread | | | Date | Wed, 09 Jun 2021 13:52:46 -0700 | From | Chris Goldsworthy <> | Subject | Re: [PATCH v2] mm: fs: invalidate bh_lrus for only cold path |
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On 2021-06-02 15:45, Minchan Kim wrote: > On Tue, Jun 01, 2021 at 04:15:40PM -0700, Andrew Morton wrote: >> On Tue, 1 Jun 2021 07:54:25 -0700 Minchan Kim <minchan@kernel.org> >> wrote: >> >> > kernel test robot reported the regression of fio.write_iops[1] >> > with [2]. >> > >> > Since lru_add_drain is called frequently, invalidate bh_lrus >> > there could increase bh_lrus cache miss ratio, which needs >> > more IO in the end. >> > >> > This patch moves the bh_lrus invalidation from the hot path( >> > e.g., zap_page_range, pagevec_release) to cold path(i.e., >> > lru_add_drain_all, lru_cache_disable). >> >> This code is starting to hurt my brain. >> >> What are the locking/context rules for invalidate_bh_lrus_cpu()? > > >> AFAICT it offers no protection against two CPUs concurrently running >> __invalidate_bh_lrus() against the same bh_lru. > > The lru_add_drain_per_cpu will run on per-cpu since it's per-cpu work > and invalidate_bh_lrus_cpu will run under bh_lru_lock so I couldn't > imagine that race can happen. > >> >> So when CONFIG_SMP=y, invalidate_bh_lrus_cpu() must always and only be >> run on the cpu which owns the bh_lru. In which case why does it have >> the `cpu' arg? > > I just wanted to express both lru_add_drain_cpu and > invalidate_bh_lrus_cpu > in lru_add_and_bh_lrus_drain run in the same cpu but look like a bad > idea > since it makes people confused. Let me remove the cpu argument from > invalidate_bh_lrus_cpu. > >> >> Your new lru_add_and_bh_lrus_drain() follows these rules by calling >> invalidate_bh_lrus_cpu() from a per-cpu worker or when CONFIG_SMP=n. >> >> I think. It's all as clear as mud and undocumented. Could you please >> take a look at this? Comment the locking/context rules thoroughly and >> check that they are being followed? Not forgetting cpu hotplug... >> See if >> there's a way of simplifying/clarifying the code? >> >> The fact that swap.c has those #ifdef CONFIG_SMPs in there is a hint >> that we're doing something wrong (or poorly) in there. Perhaps that's >> unavoidable because of all the fancy footwork in >> __lru_add_drain_all(). >> > > Hopefully, this is better. > > From 8d58e7ade3ed6c080995dec1395b1e130b3d16b3 Mon Sep 17 00:00:00 2001 > From: Minchan Kim <minchan@kernel.org> > Date: Tue, 25 May 2021 08:19:17 -0700 > Subject: [PATCH] mm: fs: invalidate bh_lrus for only cold path > > kernel test robot reported the regression of fio.write_iops[1] > with [2]. > > Since lru_add_drain is called frequently, invalidate bh_lrus > there could increase bh_lrus cache miss ratio, which needs > more IO in the end. > > This patch moves the bh_lrus invalidation from the hot path( > e.g., zap_page_range, pagevec_release) to cold path(i.e., > lru_add_drain_all, lru_cache_disable). > > [1] > https://lore.kernel.org/lkml/20210520083144.GD14190@xsang-OptiPlex-9020/ > [2] 8cc621d2f45d, mm: fs: invalidate BH LRU during page migration > Reported-by: kernel test robot <oliver.sang@intel.com> > Signed-off-by: Minchan Kim <minchan@kernel.org> > --- > fs/buffer.c | 8 ++++++-- > include/linux/buffer_head.h | 4 ++-- > mm/swap.c | 19 ++++++++++++++++--- > 3 files changed, 24 insertions(+), 7 deletions(-) > > diff --git a/fs/buffer.c b/fs/buffer.c > index 673cfbef9eec..bdaffed39030 100644 > --- a/fs/buffer.c > +++ b/fs/buffer.c > @@ -1487,12 +1487,16 @@ void invalidate_bh_lrus(void) > } > EXPORT_SYMBOL_GPL(invalidate_bh_lrus); > > -void invalidate_bh_lrus_cpu(int cpu) > +/* > + * It's called from workqueue context so we need a bh_lru_lock to > close > + * the race with preemption/irq. > + */ > +void invalidate_bh_lrus_cpu(void) > { > struct bh_lru *b; > > bh_lru_lock(); > - b = per_cpu_ptr(&bh_lrus, cpu); > + b = this_cpu_ptr(&bh_lrus); > __invalidate_bh_lrus(b); > bh_lru_unlock(); > } > diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h > index e7e99da31349..b04d34bab124 100644 > --- a/include/linux/buffer_head.h > +++ b/include/linux/buffer_head.h > @@ -194,7 +194,7 @@ void __breadahead_gfp(struct block_device *, > sector_t block, unsigned int size, > struct buffer_head *__bread_gfp(struct block_device *, > sector_t block, unsigned size, gfp_t gfp); > void invalidate_bh_lrus(void); > -void invalidate_bh_lrus_cpu(int cpu); > +void invalidate_bh_lrus_cpu(void); > bool has_bh_in_lru(int cpu, void *dummy); > struct buffer_head *alloc_buffer_head(gfp_t gfp_flags); > void free_buffer_head(struct buffer_head * bh); > @@ -408,7 +408,7 @@ static inline int inode_has_buffers(struct inode > *inode) { return 0; } > static inline void invalidate_inode_buffers(struct inode *inode) {} > static inline int remove_inode_buffers(struct inode *inode) { return > 1; } > static inline int sync_mapping_buffers(struct address_space *mapping) > { return 0; } > -static inline void invalidate_bh_lrus_cpu(int cpu) {} > +static inline void invalidate_bh_lrus_cpu(void) {} > static inline bool has_bh_in_lru(int cpu, void *dummy) { return 0; } > #define buffer_heads_over_limit 0 > > diff --git a/mm/swap.c b/mm/swap.c > index 1958d5feb148..4d9ec3c3c5a9 100644 > --- a/mm/swap.c > +++ b/mm/swap.c > @@ -642,7 +642,6 @@ void lru_add_drain_cpu(int cpu) > pagevec_lru_move_fn(pvec, lru_lazyfree_fn); > > activate_page_drain(cpu); > - invalidate_bh_lrus_cpu(cpu); > } > > /** > @@ -725,6 +724,20 @@ void lru_add_drain(void) > local_unlock(&lru_pvecs.lock); > } > > +/* > + * It's called from per-cpu workqueue context in SMP case so > + * lru_add_drain_cpu and invalidate_bh_lrus_cpu should run on > + * the same cpu. It shouldn't be a problem in !SMP case since > + * the core is only one and the locks will disable preemption. > + */ > +static void lru_add_and_bh_lrus_drain(void) > +{ > + local_lock(&lru_pvecs.lock); > + lru_add_drain_cpu(smp_processor_id()); > + local_unlock(&lru_pvecs.lock); > + invalidate_bh_lrus_cpu(); > +} > + > void lru_add_drain_cpu_zone(struct zone *zone) > { > local_lock(&lru_pvecs.lock); > @@ -739,7 +752,7 @@ static DEFINE_PER_CPU(struct work_struct, > lru_add_drain_work); > > static void lru_add_drain_per_cpu(struct work_struct *dummy) > { > - lru_add_drain(); > + lru_add_and_bh_lrus_drain(); > } > > /* > @@ -880,7 +893,7 @@ void lru_cache_disable(void) > */ > __lru_add_drain_all(true); > #else > - lru_add_drain(); > + lru_add_and_bh_lrus_drain(); > #endif > }
Hi Minchan,
This looks good to me. Feel free to add:
Reviewed-by: Chris Goldsworthy <cgoldswo@codeaurora.org>
Thanks,
Chris.
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