RE: [PATCH v4 2/2] pid: Remove pidhash
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- To: Rik van Riel <riel@xxxxxxxxxxx>, lkp <lkp@xxxxxxxxx>, Gargi Sharma <gs051095@xxxxxxxxx>
- Subject: RE: [PATCH v4 2/2] pid: Remove pidhash
- From: "Luck, Tony" <tony.luck@xxxxxxxxx>
- Date: Wed, 11 Oct 2017 21:22:08 +0000
- Accept-language: en-US
- Cc: "kbuild-all@xxxxxx" <kbuild-all@xxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, "julia.lawall@xxxxxxx" <julia.lawall@xxxxxxx>, "akpm@xxxxxxxxxxxxxxxxxxxx" <akpm@xxxxxxxxxxxxxxxxxxxx>, "mingo@xxxxxxxxxx" <mingo@xxxxxxxxxx>, "pasha.tatashin@xxxxxxxxxx" <pasha.tatashin@xxxxxxxxxx>, "ktkhai@xxxxxxxxxxxxx" <ktkhai@xxxxxxxxxxxxx>, "oleg@xxxxxxxxxx" <oleg@xxxxxxxxxx>, "ebiederm@xxxxxxxxxxxx" <ebiederm@xxxxxxxxxxxx>, "hch@xxxxxxxxxxxxx" <hch@xxxxxxxxxxxxx>, linux-ia64 <linux-ia64@xxxxxxxxxxxxxxx>
- Dlp-product: dlpe-windows
- Dlp-reaction: no-action
- Dlp-version: 11.0.0.116
- In-reply-to: <1507731304.21121.185.camel@surriel.com>
- References: <201710111737.ZHqmaPa5%fengguang.wu@intel.com> <1507731304.21121.185.camel@surriel.com>
> 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
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