linux-kernel - Re: [PATCH v7 3/4] ipc: Allow boot time extension of IPCMNI from 32k to 2M

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Date:   Mon, 7 May 2018 20:04:54 -0400
From:   Waiman Long <longman@...hat.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>, tglx@...utronix.de,
        arnd@...db.de, Matthew Wilcox <willy@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH v7 3/4] ipc: Allow boot time extension of IPCMNI from 32k
 to 2M

On 05/07/2018 07:17 PM, Luis R. Rodriguez wrote:
> On Mon, May 07, 2018 at 04:59:11PM -0400, Waiman Long wrote:
>> diff --git a/ipc/ipc_sysctl.c b/ipc/ipc_sysctl.c
>> index 49f9bf4..d62335f 100644
>> --- a/ipc/ipc_sysctl.c
>> +++ b/ipc/ipc_sysctl.c
>> @@ -120,7 +120,8 @@ static int proc_ipc_sem_dointvec(struct ctl_table *table, int write,
>>  static int zero;
>>  static int one = 1;
>>  static int int_max = INT_MAX;
>> -static int ipc_mni = IPCMNI;
>> +int ipc_mni __read_mostly = IPCMNI;
>> +int ipc_mni_shift __read_mostly = IPCMNI_SHIFT;
>>  
>>  static struct ctl_table ipc_kern_table[] = {
>>  	{
> Is use of ipc_mni and ipc_mni_shift a hot path? As per Christoph Lameter,
> its use should be reserved for data that is actually used frequently in hot
> paths, and typically this was done after performance traces reveal contention
> because a neighboring variable was frequently written to [0]. These would also
> be tightly packed, to reduce the number of cachelines needed to execute a
> critical path, so we should be selective about what variables use it.
>
> Your commit log does not describe why you'd use __read_mostly here. It would
> be useful if it did.
>
> [0] https://lkml.kernel.org/r/alpine.DEB.2.11.1504301343190.28879@gentwo.org
I used __read_mostly to reduce the performance impact of transitioning
from a constant to a variable. But you are right, their use are probably
not in a hot path. So even the use of regular variables shouldn't show
any noticeable performance difference. I can take that out in the my
next version after I gather enough feedback.

Cheers,
Longman