On Tue, Dec 12, 2023 at 8:18 PM Chengming Zhou
<zhouchengming@xxxxxxxxxxxxx> wrote:
>
> In the !zpool_can_sleep_mapped() case such as zsmalloc, we need to first
> copy the entry->handle memory to a temporary memory, which is allocated
> using kmalloc.
>
> Obviously we can reuse the per-compressor dstmem to avoid allocating
> every time, since it's percpu-compressor and protected in mutex.
You are trading more memory for faster speed.
Per-cpu data structure does not come free. It is expensive in terms of
memory on a big server with a lot of CPU. Think more than a few
hundred CPU. On the big servers, we might want to disable this
optimization to save a few MB RAM, depending on the gain of this
optimization.
Do we have any benchmark suggesting how much CPU overhead or latency
this per-CPU page buys us, compared to using kmalloc?
Chris
>
> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>
> ---
> mm/zswap.c | 29 +++++++++--------------------
> 1 file changed, 9 insertions(+), 20 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 7ee54a3d8281..edb8b45ed5a1 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1772,9 +1772,9 @@ bool zswap_load(struct folio *folio)
> struct zswap_entry *entry;
> struct scatterlist input, output;
> struct crypto_acomp_ctx *acomp_ctx;
> - u8 *src, *dst, *tmp;
> + unsigned int dlen = PAGE_SIZE;
> + u8 *src, *dst;
> struct zpool *zpool;
> - unsigned int dlen;
> bool ret;
>
> VM_WARN_ON_ONCE(!folio_test_locked(folio));
> @@ -1796,27 +1796,18 @@ bool zswap_load(struct folio *folio)
> goto stats;
> }
>
> - zpool = zswap_find_zpool(entry);
> - if (!zpool_can_sleep_mapped(zpool)) {
> - tmp = kmalloc(entry->length, GFP_KERNEL);
> - if (!tmp) {
> - ret = false;
> - goto freeentry;
> - }
> - }
> -
> /* decompress */
> - dlen = PAGE_SIZE;
> - src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
> + acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> + mutex_lock(acomp_ctx->mutex);
>
> + zpool = zswap_find_zpool(entry);
> + src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
> if (!zpool_can_sleep_mapped(zpool)) {
> - memcpy(tmp, src, entry->length);
> - src = tmp;
> + memcpy(acomp_ctx->dstmem, src, entry->length);
> + src = acomp_ctx->dstmem;
> zpool_unmap_handle(zpool, entry->handle);
> }
>
> - acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> - mutex_lock(acomp_ctx->mutex);
> sg_init_one(&input, src, entry->length);
> sg_init_table(&output, 1);
> sg_set_page(&output, page, PAGE_SIZE, 0);
> @@ -1827,15 +1818,13 @@ bool zswap_load(struct folio *folio)
>
> if (zpool_can_sleep_mapped(zpool))
> zpool_unmap_handle(zpool, entry->handle);
> - else
> - kfree(tmp);
>
> ret = true;
> stats:
> count_vm_event(ZSWPIN);
> if (entry->objcg)
> count_objcg_event(entry->objcg, ZSWPIN);
> -freeentry:
> +
> spin_lock(&tree->lock);
> if (ret && zswap_exclusive_loads_enabled) {
> zswap_invalidate_entry(tree, entry);
>
> --
> b4 0.10.1
