Date: Tue, 9 Jan 2007 07:56:33 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Andrew Morton <akpm@osdl.org>
Cc: vatsa@in.ibm.com, Oleg Nesterov <oleg@tv-sign.ru>,
       David Howells <dhowells@redhat.com>,
       Christoph Hellwig <hch@infradead.org>,
       Linus Torvalds <torvalds@osdl.org>, linux-kernel@vger.kernel.org,
       Gautham shenoy <ego@in.ibm.com>
Subject: Re: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist
Message-ID: <20070109065633.GB8115@elte.hu>
References: <20070106154506.GC24274@in.ibm.com> <20070106163035.GA2948@tv-sign.ru> <20070106163851.GA13579@in.ibm.com> <20070106111117.54bb2307.akpm@osdl.org> <20070107110013.GD13579@in.ibm.com> <20070107115957.6080aa08.akpm@osdl.org> <20070107210139.GA2332@tv-sign.ru> <20070108155428.d76f3b73.akpm@osdl.org> <20070109050417.GC589@in.ibm.com> <20070108212656.ca77a3ba.akpm@osdl.org>
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* Andrew Morton <akpm@osdl.org> wrote:

> > I would be happy to be corrected if the above impression of 
> > freeze_processes() is corrected ..
> 
> It could be that the freezer needs a bit of work for this application. 
> Obviously we're not interested in the handling of disk I/O, so we'd 
> really like to do a simple 
> try_to_freeze_tasks(FREEZER_USER_SPACE|FREEZER_KERNEL_THREADS), but 
> the code isn't set up to do that (it should be).  The other non-swsusp 
> callers probably want this change as well.  But that's all a minor 
> matter.

yes. The freezer does the fundamentally right thing: it stops all tasks 
in user-space or waits for them to return back to user-space to stop 
them there, or if it's a pure kernel-space task it waits until that 
kernel-space task voluntarily stop.

Once the system is in such a state, and all processing has been stopped, 
all of the kernel's data structures are completely 'unused', and we can:

- patch the kernel freely (kprobes)

- save+stop the kernel (sw-suspend) 

- remove a CPU (CPU hotplug and suspend)

- (We could also use this mechanism to live-migrate the kernel to 
   another system btw., possibly useful for containers)

- (We could also use this mechanism to create a live snapshot of a 
   running kernel, together with an LVM snapshot of filesystem state, 
   for possible restore point later on.)

It is a very powerful mechanism that has really nice properties - we 
should work on this one shared infrastructure instead of adding zillions 
of per-subsystem CPU hotplug locks.

	Ingo
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