linux-kernel - Re: [PATCH v2 06/33] Documentation, x86: Documentation for Intel resource allocation user interface

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Date:   Thu, 8 Sep 2016 15:45:14 -0700
From:   Shaohua Li <shli@...com>
To:     Fenghua Yu <fenghua.yu@...el.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <h.peter.anvin@...el.com>,
        Ingo Molnar <mingo@...e.hu>, Tony Luck <tony.luck@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>, Borislav Petkov <bp@...e.de>,
        Stephane Eranian <eranian@...gle.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        "David Carrillo-Cisneros" <davidcc@...gle.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        Sai Prakhya <sai.praneeth.prakhya@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v2 06/33] Documentation, x86: Documentation for Intel
 resource allocation user interface

On Thu, Sep 08, 2016 at 06:17:47PM -0700, Fenghua Yu wrote:
> On Thu, Sep 08, 2016 at 03:01:20PM -0700, Shaohua Li wrote:
> > On Thu, Sep 08, 2016 at 02:57:00AM -0700, Fenghua Yu wrote:
> > > From: Fenghua Yu <fenghua.yu@...el.com>
> > > 
> > > The documentation describes user interface of how to allocate resource
> > > in Intel RDT.
> > > 
> > > Please note that the documentation covers generic user interface. Current
> > > patch set code only implemente CAT L3. CAT L2 code will be sent later.
> > > 
> > > Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
> > > Reviewed-by: Tony Luck <tony.luck@...el.com>
> > > ---
> > >  Documentation/x86/intel_rdt_ui.txt | 164 +++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 164 insertions(+)
> > >  create mode 100644 Documentation/x86/intel_rdt_ui.txt
> > > 
> > > diff --git a/Documentation/x86/intel_rdt_ui.txt b/Documentation/x86/intel_rdt_ui.txt
> > > new file mode 100644
> > > index 0000000..27de386
> > > --- /dev/null
> > > +++ b/Documentation/x86/intel_rdt_ui.txt
> > > @@ -0,0 +1,164 @@
> > > +User Interface for Resource Allocation in Intel Resource Director Technology
> > > +
> > > +Copyright (C) 2016 Intel Corporation
> > > +
> > > +Fenghua Yu <fenghua.yu@...el.com>
> > > +Tony Luck <tony.luck@...el.com>
> > > +
> > > +This feature is enabled by the CONFIG_INTEL_RDT Kconfig and the
> > > +X86 /proc/cpuinfo flag bits "rdt", "cat_l3" and "cdp_l3".
> > > +
> > > +To use the feature mount the file system:
> > > +
> > > + # mount -t resctrl resctrl [-o cdp,verbose] /sys/fs/resctrl
> > > +
> > > +mount options are:
> > > +
> > > +"cdp": Enable code/data prioritization in L3 cache allocations.
> > > +
> > > +"verbose": Output more info in the "info" file under info directory
> > > +	and in dmesg. This is mainly for debug.
> > > +
> > > +
> > > +Resource groups
> > > +---------------
> > > +Resource groups are represented as directories in the resctrl file
> > > +system. The default group is the root directory. Other groups may be
> > > +created as desired by the system administrator using the "mkdir(1)"
> > > +command, and removed using "rmdir(1)".
> > > +
> > > +There are three files associated with each group:
> > > +
> > > +"tasks": A list of tasks that belongs to this group. Tasks can be
> > > +	added to a group by writing the task ID to the "tasks" file
> > > +	(which will automatically remove them from the previous
> > > +	group to which they belonged). New tasks created by fork(2)
> > > +	and clone(2) are added to the same group as their parent.
> > > +	If a pid is not in any sub partition, it is in root partition
> > > +	(i.e. default partition).
> > Hi Fenghua,
> > 
> > Will you add a 'procs' interface to allow move a process into a group? Using
> > the 'tasks' interface to move process is inconvenient and has race conditions
> > (eg, some new threads could be escaped).
> 
> We don't plan to add a 'procs' interface for rdtgroup. We only use resctrl
> interface to allocate resources.
> 
> Why the "tasks" is inconvenient? If sysadmin wants to allocte a portion of L3
> for a pid, the operation in resctl is to write the pid to a "tasks". While
> in 'procs', the operation is to write a partition to a pid. If considering
> convenience, they are same, right?
> 
> A thread uses either default partition (in root dir) or a sub partition (in
> sub-directory). Sysadmin can control that. Kernel handles race condition.
> Any issue with that?

I don't mean writing the 'tasks' file is inconvenient. So to move a process to
a group, we do:
1. get all thread pid of the process
2. write every pid to 'tasks'

this is inconvenient. And if a new thread is created between 1 and 2, we don't
put the thread to the group. Am I missing anything?

Thanks,
Shaohua