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Date:	Sun, 2 Feb 2014 22:50:32 +0600
From:	Rakib Mullick <rakib.mullick@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Do we really need curr_target in signal_struct ?

On Sat, Feb 1, 2014 at 10:51 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 02/01, Rakib Mullick wrote:
>>
>> On Thu, Jan 30, 2014 at 1:02 PM, Rakib Mullick <rakib.mullick@...il.com> wrote:
>> > On Thu, Jan 30, 2014 at 12:32 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> >> On 01/29, Rakib Mullick wrote:
>> >
>> >>> Are you thinking that , since things are not broken, then we shouldn't
>> >>> try to do anything?
>> >>
>> >> Hmm. No.
>> >>
>> >> I am thinking that, since you misunderstood the purpose of ->curr_target,
>> >> I should probably try to argue with your patch which blindly removes this
>> >> optimization ?
>> >>
>> > Since the optimization (usages of ->curr_target) isn't perfect, so there's
>> > chance of being misunderstood. This optimization is misleading too (I think),
>> > cause curr_target don't have anything to do with wants_signal()
>
> can't understand... curr_target is obviously connected to wants_signal() ?
No. I meant, curr_target doesn't makes sure that it really wants the
signal, it's likely
choice.

> As I already said it caches the last wants_signal(t) thread?
Yes, you said. But, gets messed up at exit path, not useful everytime.
If fails, p gets checked twice.

>> > and
>> >  ->curr_target is used only for this optimization and to get this optimization
>> > needs to maintain it properly, and this maintenance does have cost and if
>> > we don't get benefited too much, then it doesn't worth it (my pov).
>
> but you need to prove this somehow.
>
Right, I'll try, I'll do it as per my understanding and everyone might
not agree.

>> I took a look and found that using while_each_thread()
>> can make things better than current.
>
> Why?
>
using while_each_thread() we can start thread traversing from p, which
is a likely
pick and also gets away from redundant checking of p.

>> What do you think?
>
> The patch is technically wrong, a group-wide signal doesn't check all
> threads after this change.
If group is empty, we don't need to check other than t.

> And I do not understand why complete_signal()
> still updates ->curr_target.
Didn't want to remove it blindly :-/

> Plus thread_group_empty() doesn't buy too
> much if we use while_each_thread(). But this doesn't really matter, easy
> to fix.
>
> Rakib. It is not that I like ->curr_target very much. But let me repeat,
> if you want to remove this optimization you need to explain why it doesn't
> make sense. You claim that this "can make things better" without any
> argument.
>
Well, I had other way of looking at it and didn't find any real usages
of ->curr_target and got confused though the code comment in curr_target.

> As for me - I simply do not know. This logic is very old, I am not even
> sure that the current usage of ->curr_signal matches the original intent.
> But it can obviously help in some cases, and you need to explain why we
> do not care.
>
Actually, this is exactly what i wanted to know. What is the purpose
of ->curr_signal, *do we really need it?* If I knew or seen any reason
I'd have never asked this question. You guys knows this domain better than
me, that's why I asked. Git log didn't help much, neither it's documentation
. As a result, I asked and thought about cleaning it up, (as i rarely do if
it meets my capability of course), so appended a sort of rough patch.

> So I won't argue if you submit the technically correct patch, but you
> need to convince Andrew or Ingo to take it. I think the right change in
> complete_signal() is something like below. It can be even simpler and use
> the single do/while loop, but then we need to check "group" inside that
> loop. With the change below ->curr_target can be simply removed.
>
I'll try to make points to convince Andrew or Ingo, I'll try to show
up my points,
and the rest will be upon them.

And here's what i think about ->curr_target removal (as per my understanding):

 a) ->curr_target is only might be useful in group wide case.
 b) Usually signals are sent particularly to a thread so ->curr_signal
 doesn't makes sense.
 c) If ->curr_signal pointed thread is killed, curr_signal points to
the next thread,
but nothing can make sure that it's a right pick.
 d) also current in implementation p is checked twice.
 e) One use case of ->curr_signal is, it always points to the lastly
created thread,
as it's a better candidate for want_signal cause newly created thread don't
usually have any signal pending. We can acheive the same without ->curr_signal
by traversing thread group from the lastly created thread.

So, this is what I think. Let me know if these reason's looks reasonable to you,
cause before Ingo or Andrew taking it, it requires your ack.

Thanks,
Rakib.
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