linux-kernel - Re: [PATCH v1 1/2] padata: downgrade padata_do_multithreaded to serial execution for non-SMP

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Message-Id: <05F7392A-FCC9-48E3-92B3-3A43A9D0C987@linux.dev>
Date: Thu, 15 Feb 2024 18:49:46 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Gang Li <gang.li@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Steffen Klassert <steffen.klassert@...unet.com>,
 Daniel Jordan <daniel.m.jordan@...cle.com>, Jane Chu <jane.chu@...cle.com>,
 "Paul E . McKenney" <paulmck@...nel.org>,
 Randy Dunlap <rdunlap@...radead.org>, kernel test robot <lkp@...el.com>,
 Gang Li <ligang.bdlg@...edance.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH v1 1/2] padata: downgrade padata_do_multithreaded to serial execution for non-SMP



> On Feb 13, 2024, at 23:15, Gang Li <gang.li@...ux.dev> wrote:
> 
> 
> 
>> On 2024/2/13 22:52, Muchun Song wrote:
>>> 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@...radead.org>
>>> Closes: https://lore.kernel.org/lkml/ec5dc528-2c3c-4444-9e88-d2c48395b433@infradead.org/
>>> Reported-by: kernel test robot <lkp@...el.com>
>>> Closes: https://lore.kernel.org/oe-kbuild-all/202402020454.6EPkP1hi-lkp@intel.com/
>>> Signed-off-by: Gang Li <ligang.bdlg@...edance.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.
> 
> Only CRYPTO_PCRYPT, HUGETLBFS and DEFERRED_STRUCT_PAGE_INIT select
> PADATA. If drop this dependence, hugetlb init parallelization may not
> work at all.

Oh, right. In this case, maybe current choice is better.

> 
> Maybe we can set PADATA enabled on default?
> 
>>>       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