From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ravikiran G Thirumalai Subject: Re: [patch 3/4] net: Percpufy frequently used variables -- proto.sockets_allocated Date: Thu, 2 Feb 2006 19:05:47 -0800 Message-ID: <20060203030547.GB3612@localhost.localdomain> References: <20060126185649.GB3651@localhost.localdomain> <20060126190357.GE3651@localhost.localdomain> <43D9DFA1.9070802@cosmosbay.com> <20060127195227.GA3565@localhost.localdomain> <20060127121602.18bc3f25.akpm@osdl.org> <20060127224433.GB3565@localhost.localdomain> <20060127150106.38b9e041.akpm@osdl.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: dada1@cosmosbay.com, davem@davemloft.net, linux-kernel@vger.kernel.org, shai@scalex86.org, netdev@vger.kernel.org, pravins@calsoftinc.com, bcrl@kvack.org Return-path: To: Andrew Morton Content-Disposition: inline In-Reply-To: <20060127150106.38b9e041.akpm@osdl.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Fri, Jan 27, 2006 at 03:01:06PM -0800, Andrew Morton wrote: > Ravikiran G Thirumalai wrote: > > > > > > > > If the benchmarks say that we need to. If we cannot observe any problems > > > in testing of existing code and if we can't demonstrate any benefit from > > > the patched code then one option is to go off and do something else ;) > > > > We first tried plain per-CPU counters for memory_allocated, found that reads > > on memory_allocated was causing cacheline transfers, and then > > switched over to batching. So batching reads is useful. To avoid > > inaccuracy, we can maybe change percpu_counter_init to: > > > > void percpu_counter_init(struct percpu_counter *fbc, int maxdev) > > > > the percpu batching limit would then be maxdev/num_possible_cpus. One would > > use batching counters only when both reads and writes are frequent. With > > the above scheme, we would go fetch cachelines from other cpus for read > > often only on large cpu counts, which is not any worse than the global > > counter alternative, but it would still be beneficial on smaller machines, > > without sacrificing a pre-set deviation. > > > > Comments? > > Sounds sane. > Here's an implementation which delegates tuning of batching to the user. We don't really need local_t at all as percpu_counter_mod is not safe against interrupts and softirqs as it is. If we have a counter which could be modified in process context and irq/bh context, we just have to use a wrapper like percpu_counter_mod_bh which will just disable and enable bottom halves. Reads on the counters are safe as they are atomic_reads, and the cpu local variables are always accessed by that cpu only. (PS: the maxerr for ext2/ext3 is just guesstimate) Comments? Index: linux-2.6.16-rc1mm4/include/linux/percpu_counter.h =================================================================== --- linux-2.6.16-rc1mm4.orig/include/linux/percpu_counter.h 2006-02-02 11:18:54.000000000 -0800 +++ linux-2.6.16-rc1mm4/include/linux/percpu_counter.h 2006-02-02 18:29:46.000000000 -0800 @@ -16,24 +16,32 @@ struct percpu_counter { atomic_long_t count; + int percpu_batch; long *counters; }; -#if NR_CPUS >= 16 -#define FBC_BATCH (NR_CPUS*2) -#else -#define FBC_BATCH (NR_CPUS*4) -#endif -static inline void percpu_counter_init(struct percpu_counter *fbc) +/* + * Choose maxerr carefully. maxerr/num_possible_cpus indicates per-cpu batching + * Set maximum tolerance for better performance on large systems. + */ +static inline void percpu_counter_init(struct percpu_counter *fbc, + unsigned int maxerr) { atomic_long_set(&fbc->count, 0); - fbc->counters = alloc_percpu(long); + fbc->percpu_batch = maxerr/num_possible_cpus(); + if (fbc->percpu_batch) { + fbc->counters = alloc_percpu(long); + if (!fbc->counters) + fbc->percpu_batch = 0; + } + } static inline void percpu_counter_destroy(struct percpu_counter *fbc) { - free_percpu(fbc->counters); + if (fbc->percpu_batch) + free_percpu(fbc->counters); } void percpu_counter_mod(struct percpu_counter *fbc, long amount); @@ -63,7 +71,8 @@ struct percpu_counter { long count; }; -static inline void percpu_counter_init(struct percpu_counter *fbc) +static inline void percpu_counter_init(struct percpu_counter *fbc, + unsigned int maxerr) { fbc->count = 0; } Index: linux-2.6.16-rc1mm4/mm/swap.c =================================================================== --- linux-2.6.16-rc1mm4.orig/mm/swap.c 2006-01-29 20:20:20.000000000 -0800 +++ linux-2.6.16-rc1mm4/mm/swap.c 2006-02-02 18:36:21.000000000 -0800 @@ -470,13 +470,20 @@ static int cpu_swap_callback(struct noti #ifdef CONFIG_SMP void percpu_counter_mod(struct percpu_counter *fbc, long amount) { - long count; long *pcount; - int cpu = get_cpu(); + long count; + int cpu; + /* Slow mode */ + if (unlikely(!fbc->percpu_batch)) { + atomic_long_add(amount, &fbc->count); + return; + } + + cpu = get_cpu(); pcount = per_cpu_ptr(fbc->counters, cpu); count = *pcount + amount; - if (count >= FBC_BATCH || count <= -FBC_BATCH) { + if (count >= fbc->percpu_batch || count <= -fbc->percpu_batch) { atomic_long_add(count, &fbc->count); count = 0; } Index: linux-2.6.16-rc1mm4/fs/ext2/super.c =================================================================== --- linux-2.6.16-rc1mm4.orig/fs/ext2/super.c 2006-02-02 18:30:28.000000000 -0800 +++ linux-2.6.16-rc1mm4/fs/ext2/super.c 2006-02-02 18:36:39.000000000 -0800 @@ -610,6 +610,7 @@ static int ext2_fill_super(struct super_ int db_count; int i, j; __le32 features; + int maxerr; sbi = kmalloc(sizeof(*sbi), GFP_KERNEL); if (!sbi) @@ -835,9 +836,14 @@ static int ext2_fill_super(struct super_ printk ("EXT2-fs: not enough memory\n"); goto failed_mount; } - percpu_counter_init(&sbi->s_freeblocks_counter); - percpu_counter_init(&sbi->s_freeinodes_counter); - percpu_counter_init(&sbi->s_dirs_counter); + + if (num_possible_cpus() <= 16 ) + maxerr = 256; + else + maxerr = 1024; + percpu_counter_init(&sbi->s_freeblocks_counter, maxerr); + percpu_counter_init(&sbi->s_freeinodes_counter, maxerr); + percpu_counter_init(&sbi->s_dirs_counter, maxerr); bgl_lock_init(&sbi->s_blockgroup_lock); sbi->s_debts = kmalloc(sbi->s_groups_count * sizeof(*sbi->s_debts), GFP_KERNEL); Index: linux-2.6.16-rc1mm4/fs/ext3/super.c =================================================================== --- linux-2.6.16-rc1mm4.orig/fs/ext3/super.c 2006-02-02 18:30:28.000000000 -0800 +++ linux-2.6.16-rc1mm4/fs/ext3/super.c 2006-02-02 18:38:10.000000000 -0800 @@ -1353,6 +1353,7 @@ static int ext3_fill_super (struct super int i; int needs_recovery; __le32 features; + int maxerr; sbi = kmalloc(sizeof(*sbi), GFP_KERNEL); if (!sbi) @@ -1578,9 +1579,14 @@ static int ext3_fill_super (struct super goto failed_mount; } - percpu_counter_init(&sbi->s_freeblocks_counter); - percpu_counter_init(&sbi->s_freeinodes_counter); - percpu_counter_init(&sbi->s_dirs_counter); + if (num_possible_cpus() <= 16) + maxerr = 256; + else + maxerr = 1024; + + percpu_counter_init(&sbi->s_freeblocks_counter, maxerr); + percpu_counter_init(&sbi->s_freeinodes_counter, maxerr); + percpu_counter_init(&sbi->s_dirs_counter, maxerr); bgl_lock_init(&sbi->s_blockgroup_lock); for (i = 0; i < db_count; i++) {