linux-kernel - RE: [PATCH v4 2/2] pid: Remove pidhash

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Date:   Wed, 11 Oct 2017 21:22:08 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Rik van Riel <riel@...riel.com>, lkp <lkp@...el.com>,
        Gargi Sharma <gs051095@...il.com>
CC:     "kbuild-all@...org" <kbuild-all@...org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "julia.lawall@...6.fr" <julia.lawall@...6.fr>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "pasha.tatashin@...cle.com" <pasha.tatashin@...cle.com>,
        "ktkhai@...tuozzo.com" <ktkhai@...tuozzo.com>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "ebiederm@...ssion.com" <ebiederm@...ssion.com>,
        "hch@...radead.org" <hch@...radead.org>,
        linux-ia64 <linux-ia64@...r.kernel.org>
Subject: RE: [PATCH v4 2/2] pid: Remove pidhash

>        DEFINE(IA64_UPID_SHIFT, 5);
>
> Grepping for IA64_UPID_SHIFT leads us to some assembly
> code implementing fsys_getpid (why is that in assembly?!):

The fast system call path has a whole host of serious restrictions on what it can
touch. See Documentation/ia64/fsys.txt.  Why is getpid() a fast system call? I think
there was some application (or perhaps benchmark) that used it a lot.

>        add r8=IA64_PID_LEVEL_OFFSET,r17
>        ;;
>        ld4 r8=[r8]                             // r8 = pid->level
>        add r17=IA64_PID_UPID_OFFSET,r17        // r17 = &pid->numbers[0]
>        ;;
>        shl r8=r8,IA64_UPID_SHIFT
>        ;;
>        add r17=r17,r8                          // r17 = &pid->numbers[pid->level]
>        ;;
>        ld4 r8=[r17]                            // r8 = pid->numbers[pid->level].nr
>        ;;
>        mov r17=0
>
> Luckily it looks like this is only referencing the first members of struct upid,
> and you are removing the last member, so I suspect you will be fine changing the IA64
> to this:
>
>        BUILD_BUG_ON(sizeof(struct upid) != 16);
>        DEFINE(IA64_UPID_SHIFT, 4);
>
> Tony, does that look ok to you?

I think so.  Respin and Cc: me on both patches in the series and I'll take it for a spin
on h/w to make sure.

-Tony