From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8) Date: Thu, 03 Jun 2010 12:05:12 +0200 Message-ID: <1275559512.27810.35287.camel@twins> 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: 8BIT 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 , James Bottomley , Arve =?ISO-8859-1?Q?Hj=F8nnev=E5g?= , Neil Brown , "tytso@mit.edu" , 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) Why would the kernel change the QoS state of a task? Why not have two interacting QoS variables, one for the task, one for the subsystem in question, and the action depends on their relative value? > > 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. Right, and I can imagine that depending on the platform details and not the device details, so we get platform hooks in the drivers, or possible up in the generic stack because I don't think NICs actually know if there are open connections.