linux-kernel - Re: [PATCH v6 5/8] padata: downgrade padata_do_multithreaded to serial execution for non-SMP

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Date: Tue, 27 Feb 2024 16:26:39 -0500
From: Daniel Jordan <daniel.m.jordan@...cle.com>
To: Gang Li <gang.li@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Jane Chu <jane.chu@...cle.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, ligang.bdlg@...edance.com
Subject: Re: [PATCH v6 5/8] padata: downgrade padata_do_multithreaded to
 serial execution for non-SMP

On Thu, Feb 22, 2024 at 10:04:18PM +0800, Gang Li wrote:
> 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.
> 
> Signed-off-by: Gang Li <ligang.bdlg@...edance.com>
> Tested-by: Paul E. McKenney <paulmck@...nel.org>
> ---
>  include/linux/padata.h | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/padata.h b/include/linux/padata.h
> index 8f418711351bc..0146daf344306 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 +190,12 @@ 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)
> +{

An early return here for zero-sized jobs is consistent with the
CONFIG_PADATA version and avoids hugetlb_pages_alloc_boot taking a lock
and flushing the tlb when there's no work to do.

With that,

Acked-by: Daniel Jordan <daniel.m.jordan@...cle.com>

> +	job->thread_fn(job->start, job->start + job->size, job->fn_arg);
> +}
> +#endif
> +
>  #endif
> -- 
> 2.20.1
>