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Date: Mon, 5 May 2014 13:32:16 +0300
From: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To: Weijie Yang <weijie.yang@...sung.com>
Cc: 'Minchan Kim' <minchan@...nel.org>,
'Andrew Morton' <akpm@...ux-foundation.org>,
'Nitin Gupta' <ngupta@...are.org>,
'Sergey Senozhatsky' <sergey.senozhatsky@...il.com>,
'Bob Liu' <bob.liu@...cle.com>,
'Dan Streetman' <ddstreet@...e.org>, weijie.yang.kh@...il.com,
heesub.shin@...sung.com,
'linux-kernel' <linux-kernel@...r.kernel.org>,
'Linux-MM' <linux-mm@...ck.org>
Subject: Re: [PATCH] zram: remove global tb_lock by using lock-free CAS
Hello Weijie,
On (05/05/14 12:01), Weijie Yang wrote:
> Currently, we use a rwlock tb_lock to protect concurrent access to
> whole zram meta table. However, according to the actual access model,
> there is only a small chance for upper user access the same table[index],
> so the current lock granularity is too big.
>
> This patch add a atomic state for every table[index] to record its access,
> by using CAS operation, protect concurrent access to the same table[index],
> meanwhile allow the maximum concurrency.
>
> On 64-bit system, it will not increase the meta table memory overhead, and
> on 32-bit system with 4K page_size, it will increase about 1MB memory overhead
> for 1GB zram. So, it is cost-efficient.
>
not sure if it worth the effort (just an idea), but can we merge `u8 flags' and
`atomic_t state' into `atomic_t flags'. then reserve, say, 1 bit for IDLE/BUSY
(ACCESS) bit and the rest for flags (there is only one at the moment - ZRAM_ZERO).
> Test result:
> (x86-64 Intel Core2 Q8400, system memory 4GB, Ubuntu 12.04,
> kernel v3.15.0-rc3, zram 1GB with 4 max_comp_streams LZO,
> take the average of 5 tests)
>
> iozone -t 4 -R -r 16K -s 200M -I +Z
>
> Test base lock-free ratio
> ------------------------------------------------------
> Initial write 1348017.60 1424141.62 +5.6%
> Rewrite 1520189.16 1652504.81 +8.7%
> Read 8294445.45 11404668.35 +37.5%
> Re-read 8134448.83 11555483.75 +42.1%
> Reverse Read 6748717.97 8394478.17 +24.4%
> Stride read 7220276.66 9372229.95 +29.8%
> Random read 7133010.06 9187221.90 +28.8%
> Mixed workload 4056980.71 5843370.85 +44.0%
> Random write 1470106.17 1608947.04 +9.4%
> Pwrite 1259493.72 1311055.32 +4.1%
> Pread 4247583.17 4652056.11 +9.5%
>
> Signed-off-by: Weijie Yang <weijie.yang@...sung.com>
> ---
>
> This patch is based on linux-next tree, commit b5c8d48bf8f42
>
> drivers/block/zram/zram_drv.c | 41 ++++++++++++++++++++++++++---------------
> drivers/block/zram/zram_drv.h | 5 ++++-
> 2 files changed, 30 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 48eccb3..8b70945
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -255,7 +255,6 @@ static struct zram_meta *zram_meta_alloc(u64 disksize)
> goto free_table;
> }
>
> - rwlock_init(&meta->tb_lock);
> return meta;
>
> free_table:
> @@ -339,12 +338,14 @@ static int zram_decompress_page(struct zram *zram, char *mem, u32 index)
> unsigned long handle;
> u16 size;
>
> - read_lock(&meta->tb_lock);
> + while(atomic_cmpxchg(&meta->table[index].state, IDLE, ACCESS) != IDLE)
> + cpu_relax();
> +
a minor nitpick, this seems to be a common for 6 places, so how about factoring
out this loop to a `static inline zram_wait_for_idle_bit(meta, index)' function
(the naming is not perfect, just as example)?
'while()' does not pass checkpatch's coding style check.
-ss
> handle = meta->table[index].handle;
> size = meta->table[index].size;
>
> if (!handle || zram_test_flag(meta, index, ZRAM_ZERO)) {
> - read_unlock(&meta->tb_lock);
> + atomic_set(&meta->table[index].state, IDLE);
> clear_page(mem);
> return 0;
> }
> @@ -355,7 +356,7 @@ static int zram_decompress_page(struct zram *zram, char *mem, u32 index)
> else
> ret = zcomp_decompress(zram->comp, cmem, size, mem);
> zs_unmap_object(meta->mem_pool, handle);
> - read_unlock(&meta->tb_lock);
> + atomic_set(&meta->table[index].state, IDLE);
>
> /* Should NEVER happen. Return bio error if it does. */
> if (unlikely(ret)) {
> @@ -376,14 +377,16 @@ static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
> struct zram_meta *meta = zram->meta;
> page = bvec->bv_page;
>
> - read_lock(&meta->tb_lock);
> + while(atomic_cmpxchg(&meta->table[index].state, IDLE, ACCESS) != IDLE)
> + cpu_relax();
> +
> if (unlikely(!meta->table[index].handle) ||
> zram_test_flag(meta, index, ZRAM_ZERO)) {
> - read_unlock(&meta->tb_lock);
> + atomic_set(&meta->table[index].state, IDLE);
> handle_zero_page(bvec);
> return 0;
> }
> - read_unlock(&meta->tb_lock);
> + atomic_set(&meta->table[index].state, IDLE);
>
> if (is_partial_io(bvec))
> /* Use a temporary buffer to decompress the page */
> @@ -461,10 +464,13 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> if (page_zero_filled(uncmem)) {
> kunmap_atomic(user_mem);
> /* Free memory associated with this sector now. */
> - write_lock(&zram->meta->tb_lock);
> + while(atomic_cmpxchg(&meta->table[index].state,
> + IDLE, ACCESS) != IDLE)
> + cpu_relax();
> +
> zram_free_page(zram, index);
> zram_set_flag(meta, index, ZRAM_ZERO);
> - write_unlock(&zram->meta->tb_lock);
> + atomic_set(&meta->table[index].state, IDLE);
>
> atomic64_inc(&zram->stats.zero_pages);
> ret = 0;
> @@ -514,12 +520,13 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> * Free memory associated with this sector
> * before overwriting unused sectors.
> */
> - write_lock(&zram->meta->tb_lock);
> + while(atomic_cmpxchg(&meta->table[index].state, IDLE, ACCESS) != IDLE)
> + cpu_relax();
> zram_free_page(zram, index);
>
> meta->table[index].handle = handle;
> meta->table[index].size = clen;
> - write_unlock(&zram->meta->tb_lock);
> + atomic_set(&meta->table[index].state, IDLE);
>
> /* Update stats */
> atomic64_add(clen, &zram->stats.compr_data_size);
> @@ -560,6 +567,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> int offset, struct bio *bio)
> {
> size_t n = bio->bi_iter.bi_size;
> + struct zram_meta *meta = zram->meta;
>
> /*
> * zram manages data in physical block size units. Because logical block
> @@ -584,9 +592,11 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> * Discard request can be large so the lock hold times could be
> * lengthy. So take the lock once per page.
> */
> - write_lock(&zram->meta->tb_lock);
> + while(atomic_cmpxchg(&meta->table[index].state,
> + IDLE, ACCESS) != IDLE)
> + cpu_relax();
> zram_free_page(zram, index);
> - write_unlock(&zram->meta->tb_lock);
> + atomic_set(&meta->table[index].state, IDLE);
> index++;
> n -= PAGE_SIZE;
> }
> @@ -804,9 +814,10 @@ static void zram_slot_free_notify(struct block_device *bdev,
> zram = bdev->bd_disk->private_data;
> meta = zram->meta;
>
> - write_lock(&meta->tb_lock);
> + while(atomic_cmpxchg(&meta->table[index].state, IDLE, ACCESS) != IDLE)
> + cpu_relax();
> zram_free_page(zram, index);
> - write_unlock(&meta->tb_lock);
> + atomic_set(&meta->table[index].state, IDLE);
> atomic64_inc(&zram->stats.notify_free);
> }
>
> diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> index 7f21c14..76b2bb5
> --- a/drivers/block/zram/zram_drv.h
> +++ b/drivers/block/zram/zram_drv.h
> @@ -61,9 +61,13 @@ enum zram_pageflags {
>
> /*-- Data structures */
>
> +#define IDLE 0
> +#define ACCESS 1
> +
> /* Allocated for each disk page */
> struct table {
> unsigned long handle;
> + atomic_t state;
> u16 size; /* object size (excluding header) */
> u8 flags;
> } __aligned(4);
> @@ -81,7 +85,6 @@ struct zram_stats {
> };
>
> struct zram_meta {
> - rwlock_t tb_lock; /* protect table */
> struct table *table;
> struct zs_pool *mem_pool;
> };
> --
> 1.7.10.4
>
>
--
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