linux-kernel - Re: [patch 00/11] ANNOUNCE: "Syslets", generic asynchronous system call support

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Message-ID: <Pine.LNX.4.64.0702140909320.7480@alien.or.mcafeemobile.com>
Date:	Wed, 14 Feb 2007 09:17:50 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Alan <alan@...rguk.ukuu.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Ulrich Drepper <drepper@...hat.com>,
	Zach Brown <zach.brown@...cle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 00/11] ANNOUNCE: "Syslets", generic asynchronous system
 call support

On Wed, 14 Feb 2007, Ingo Molnar wrote:

> 
> * Evgeniy Polyakov <johnpol@....mipt.ru> wrote:
> 
> > Let me clarify what I meant. There is only limited number of threads, 
> > which are supposed to execute blocking context, so when all they are 
> > used, main one will block too - I asked about possibility to reuse the 
> > same thread to execute queue of requests attached to it, each request 
> > can block, but if blocking issue is removed, it would be possible to 
> > return.
> 
> ah, ok, i understand your point. This is not quite possible: the 
> cachemisses are driven from schedule(), which can be arbitraily deep 
> inside arbitrary system calls. It can be in a mutex_lock() deep inside a 
> driver. It can be due to a alloc_pages() call done by a kmalloc() call 
> done from within ext3, which was called from the loopback block driver, 
> which was called from XFS, which was called from a VFS syscall.
> 
> Even if it were possible to backtrack i'm quite sure we dont want to do 
> this, for three main reasons:

IMO it'd be quite simple. We detect the service-thread full condition, 
*before* entering exec_atom and we queue the atom in an async_head request 
list. Yes, there is the chance that from the test time in sys_async_exec, 
to the time we'll end up entering exec_atom and down to schedule, one 
of the threads would become free, but IMO better that blocking 
sys_async_exec.



- Davide


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