From mboxrd@z Thu Jan 1 00:00:00 1970 From: James Bottomley Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8) Date: Thu, 03 Jun 2010 08:24:31 -0500 Message-ID: <1275571471.5914.2.camel@mulgrave.site> References: <20100527232357.6d14fdb2@lxorguk.ukuu.org.uk> <20100601135102.GA8098@srcf.ucam.org> <1275426085.21962.967.camel@mulgrave.site> <201006020024.14220.rjw@sisk.pl> <1275431816.21962.1108.camel@mulgrave.site> <1275451342.21962.1777.camel@mulgrave.site> <1275491111.2799.110.camel@mulgrave.site> <20100602214748.7742e3ae@schatten.dmk.lab> <1275511271.2799.516.camel@mulgrave.site> <20100603010607.5baf82a6@schatten.dmk.lab> <20100603110312.48a508dc@lxorguk.ukuu.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20100603110312.48a508dc@lxorguk.ukuu.org.uk> Sender: linux-kernel-owner@vger.kernel.org To: Alan Cox Cc: "Gross, Mark" , Florian Mickler , Arve =?ISO-8859-1?Q?Hj=F8nnev=E5g?= , Neil Brown , "tytso@mit.edu" , Peter Zijlstra , LKML , Thomas Gleixner , Linux OMAP Mailing List , Linux PM , "felipe.balbi@nokia.com" List-Id: linux-omap@vger.kernel.org On Thu, 2010-06-03 at 11:03 +0100, Alan Cox wrote: > > [mtg: ] This has been a pain point for the PM_QOS implementation. They change the constrain back and forth at the transaction level of the i2c driver. The pm_qos code really wasn't made to deal with such hot path use, as each such change triggers a re-computation of what the aggregate qos request is. > > That should be trivial in the usual case because 99% of the time you can > hot path > > the QoS entry changing is the latest one > there have been no other changes > If it is valid I can use the cached previous aggregate I cunningly > saved in the top QoS entry when I computed the new one > > (ie most of the time from the kernel side you have a QoS stack) It's not just the list based computation: that's trivial to fix, as you say ... the other problem is the notifier chain, because that's blocking and could be long. Could we invoke the notifier through a workqueue? It doesn't seem to have veto power, so it's pure notification, does it matter if the notice is delayed (as long as it's in order)? > > We've had a number of attempts at fixing this, but I think the proper fix is to bolt a "disable C-states > x" interface into cpu_idle that bypases pm_qos altogether. Or, perhaps add a new pm_qos API that does the equivalent operation, overriding whatever constraint is active. > > We need some of this anyway for deep power saving because there is > hardware which can't wake from soem states, which in turn means if that > device is active we need to be above the state in question. James