linux-kernel - Re: [PATCH v4 09/10] mm: zswap: Allocate pool batching resources if the crypto

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Message-ID: <CAKEwX=PmKWH4Z4Py9Jti9fcD6qCYJBBRrDF48qdmo8-i+LzzfA@mail.gmail.com>
Date: Mon, 2 Dec 2024 11:15:33 -0800
From: Nhat Pham <nphamcs@...il.com>
To: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, hannes@...xchg.org, 
	yosryahmed@...gle.com, chengming.zhou@...ux.dev, usamaarif642@...il.com, 
	ryan.roberts@....com, ying.huang@...el.com, 21cnbao@...il.com, 
	akpm@...ux-foundation.org, linux-crypto@...r.kernel.org, 
	herbert@...dor.apana.org.au, davem@...emloft.net, clabbe@...libre.com, 
	ardb@...nel.org, ebiggers@...gle.com, surenb@...gle.com, 
	kristen.c.accardi@...el.com, wajdi.k.feghali@...el.com, 
	vinodh.gopal@...el.com
Subject: Re: [PATCH v4 09/10] mm: zswap: Allocate pool batching resources if
 the crypto_alg supports batching.

On Fri, Nov 22, 2024 at 11:01 PM Kanchana P Sridhar
<kanchana.p.sridhar@...el.com> wrote:
>
> This patch does the following:
>
> 1) Modifies the definition of "struct crypto_acomp_ctx" to represent a
>    configurable number of acomp_reqs and buffers. Adds a "nr_reqs" to
>    "struct crypto_acomp_ctx" to contain the nr of resources that will be
>    allocated in the cpu onlining code.
>
> 2) The zswap_cpu_comp_prepare() cpu onlining code will detect if the
>    crypto_acomp created for the pool (in other words, the zswap compression
>    algorithm) has registered an implementation for batch_compress() and
>    batch_decompress(). If so, it will set "nr_reqs" to
>    SWAP_CRYPTO_BATCH_SIZE and allocate these many reqs/buffers, and set
>    the acomp_ctx->nr_reqs accordingly. If the crypto_acomp does not support
>    batching, "nr_reqs" defaults to 1.
>
> 3) Adds a "bool can_batch" to "struct zswap_pool" that step (2) will set to
>    true if the batching API are present for the crypto_acomp.

Why do we need this "can_batch" field? IIUC, this can be determined
from the compressor internal fields itself, no?

acomp_has_async_batching(acomp);

Is this just for convenience, or is this actually an expensive thing to compute?

>
> SWAP_CRYPTO_BATCH_SIZE is set to 8, which will be the IAA compress batching

I like a sane default value as much as the next guy, but this seems a
bit odd to me:

1. The placement of this constant/default value seems strange to me.
This is a compressor-specific value no? Why are we enforcing this
batching size at the zswap level, and uniformly at that? What if we
introduce a new batch compression algorithm...? Or am I missing
something, and this is a sane default for other compressors too?

2. Why is this value set to 8? Experimentation? Could you add some
justification in documentation?