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Message-ID: <89b3bee3-5ee3-4b75-99e3-881b759e79de@linux.dev>
Date: Tue, 13 Feb 2024 22:52:11 +0800
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MIME-Version: 1.0
Subject: Re: [PATCH v1 1/2] padata: downgrade padata_do_multithreaded to
 serial execution for non-SMP
To: Gang Li <gang.li@linux.dev>, Andrew Morton <akpm@linux-foundation.org>,
 Steffen Klassert <steffen.klassert@secunet.com>,
 Daniel Jordan <daniel.m.jordan@oracle.com>, Jane Chu <jane.chu@oracle.com>,
 "Paul E . McKenney" <paulmck@kernel.org>,
 Randy Dunlap <rdunlap@infradead.org>, kernel test robot <lkp@intel.com>,
 Gang Li <ligang.bdlg@bytedance.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <20240213111347.3189206-1-gang.li@linux.dev>
 <20240213111347.3189206-2-gang.li@linux.dev>
From: Muchun Song <muchun.song@linux.dev>
In-Reply-To: <20240213111347.3189206-2-gang.li@linux.dev>
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On 2024/2/13 19:13, Gang Li wrote:
> Randy Dunlap and kernel test robot reported a warning:
>
> ```
> WARNING: unmet direct dependencies detected for PADATA
>    Depends on [n]: SMP [=n]
>    Selected by [y]:
>    - HUGETLBFS [=y] && (X86 [=y] || SPARC64 || ARCH_SUPPORTS_HUGETLBFS [=n] || BROKEN [=n]) && (SYSFS [=y] || SYSCTL [=n])
> ```
>
> hugetlb parallelization depends on PADATA, and PADATA depends on SMP.
>
> PADATA consists of two distinct functionality: One part is
> padata_do_multithreaded which disregards order and simply divides
> tasks into several groups for parallel execution. Hugetlb
> init parallelization depends on padata_do_multithreaded.
>
> The other part is composed of a set of APIs that, while handling data in
> an out-of-order parallel manner, can eventually return the data with
> ordered sequence. Currently Only `crypto/pcrypt.c` use them.
>
> All users of PADATA of non-SMP case currently only use
> padata_do_multithreaded. It is easy to implement a serial one in
> include/linux/padata.h. And it is not necessary to implement another
> functionality unless the only user of crypto/pcrypt.c does not depend on
> SMP in the future.
>
> Fixes: a2cefb08be66 ("hugetlb: have CONFIG_HUGETLBFS select CONFIG_PADATA")
> Reported-by: Randy Dunlap <rdunlap@infradead.org>
> Closes: https://lore.kernel.org/lkml/ec5dc528-2c3c-4444-9e88-d2c48395b433@infradead.org/
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202402020454.6EPkP1hi-lkp@intel.com/
> Signed-off-by: Gang Li <ligang.bdlg@bytedance.com>
> ---
>   fs/Kconfig             |  2 +-
>   include/linux/padata.h | 13 +++++++++----
>   2 files changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/fs/Kconfig b/fs/Kconfig
> index 4a51331f172e5..7963939592d70 100644
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -261,7 +261,7 @@ menuconfig HUGETLBFS
>   	depends on X86 || SPARC64 || ARCH_SUPPORTS_HUGETLBFS || BROKEN
>   	depends on (SYSFS || SYSCTL)
>   	select MEMFD_CREATE
> -	select PADATA
> +	select PADATA if SMP

I'd like to drop this dependence since HugeTLB does not depend
on PADATA anymore. If some users take care about the kernel
image size, it also can disable PADATA individually.

>   	help
>   	  hugetlbfs is a filesystem backing for HugeTLB pages, based on
>   	  ramfs. For architectures that support it, say Y here and read
> diff --git a/include/linux/padata.h b/include/linux/padata.h
> index 8f418711351bc..7b84eb7d73e7f 100644
> --- a/include/linux/padata.h
> +++ b/include/linux/padata.h
> @@ -180,10 +180,6 @@ struct padata_instance {
>   
>   #ifdef CONFIG_PADATA
>   extern void __init padata_init(void);
> -#else
> -static inline void __init padata_init(void) {}
> -#endif
> -
>   extern struct padata_instance *padata_alloc(const char *name);
>   extern void padata_free(struct padata_instance *pinst);
>   extern struct padata_shell *padata_alloc_shell(struct padata_instance *pinst);
> @@ -194,4 +190,13 @@ extern void padata_do_serial(struct padata_priv *padata);
>   extern void __init padata_do_multithreaded(struct padata_mt_job *job);
>   extern int padata_set_cpumask(struct padata_instance *pinst, int cpumask_type,
>   			      cpumask_var_t cpumask);
> +#else
> +static inline void __init padata_init(void) {}
> +static inline void __init padata_do_multithreaded(struct padata_mt_job *job)
> +{
> +	if (job->size)

I think we could drop this check, at least now there is no users will
pass a zero of ->size to this function, and even if someone does in the
future, I think it is really a corner case, it is unnecessary to optimize
it and ->thread_fn is supporsed to handle case of zero size if it dose
pass a zero size.

Thanks.

> +		job->thread_fn(job->start, job->start + job->size, job->fn_arg);
> +}
> +#endif
> +
>   #endif