Subject: Re: [PATCH v3] pipe: use __pipe_{lock,unlock} instead of spinlock
To:     Andrew Morton <akpm@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>
Cc:     Matthew Wilcox <willy@infradead.org>,
        maobibo <maobibo@loongson.cn>,
        David Howells <dhowells@redhat.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        "Christian Brauner (Microsoft)" <brauner@kernel.org>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20230107012324.30698-1-zhanghongchen@loongson.cn>
 <9fcb3f80-cb55-9a72-0e74-03ace2408d21@loongson.cn>
 <4b140bd0-9b7f-50b5-9e3b-16d8afe52a50@loongson.cn>
 <Y8TUqcSO5VrbYfcM@casper.infradead.org> <Y8W9TR5ifZmRADLB@ZenIV>
 <20230116141608.a72015bdd8bbbedd5c50cc3e@linux-foundation.org>
From:   Hongchen Zhang <zhanghongchen@loongson.cn>
Message-ID: <90dd93c0-d0ed-50c1-9a86-dad5bb3754af@loongson.cn>
Date:   Sun, 29 Jan 2023 10:29:17 +0800
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Hi Andrew,

Sorry to reply to you so late, because I took a long holiday.

On 2023/1/17 am 6:16, Andrew Morton wrote:
> On Mon, 16 Jan 2023 21:10:37 +0000 Al Viro <viro@zeniv.linux.org.uk> wrote:
> 
>> On Mon, Jan 16, 2023 at 04:38:01AM +0000, Matthew Wilcox wrote:
>>> On Mon, Jan 16, 2023 at 11:16:13AM +0800, maobibo wrote:
>>>> Hongchen,
>>>>
>>>> I have a glance with this patch, it simply replaces with
>>>> spinlock_irqsave with mutex lock. There may be performance
>>>> improvement with two processes competing with pipe, however
>>>> for N processes, there will be complex context switches
>>>> and ipi interruptts.
>>>>
>>>> Can you find some cases with more than 2 processes competing
>>>> pipe, rather than only unixbench?
>>>
>>> What real applications have pipes with more than 1 writer & 1 reader?
>>> I'm OK with slowing down the weird cases if the common cases go faster.
>>
>> >From commit 0ddad21d3e99c743a3aa473121dc5561679e26bb:
>>      While this isn't a common occurrence in the traditional "use a pipe as a
>>      data transport" case, where you typically only have a single reader and
>>      a single writer process, there is one common special case: using a pipe
>>      as a source of "locking tokens" rather than for data communication.
>>      
>>      In particular, the GNU make jobserver code ends up using a pipe as a way
>>      to limit parallelism, where each job consumes a token by reading a byte
>>      from the jobserver pipe, and releases the token by writing a byte back
>>      to the pipe.
> 
> The author has tested this patch with Linus's test code from 0ddad21d3e
> and the results were OK
> (https://lkml.kernel.org/r/c3cbede6-f19e-3333-ba0f-d3f005e5d599@loongson.cn).
> 
> I've been stalling on this patch until Linus gets back to his desk,
> which now appears to have happened.
> 
> Hongchen, when convenient, please capture this discussion (as well as
> the testing results with Linus's sample code) in the changelog and send
> us a v4, with Linus on cc?
> 
I will send you a v4 and cc to Linus.

Thanks.
Hongchen Zhang