linux-kernel - Re: [RFC PATCH V2 06/22] x86/intel_rdt: Create pseudo-locked regions

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Message-ID: <alpine.DEB.2.21.1802200012030.1853@nanos.tec.linutronix.de>
Date:   Tue, 20 Feb 2018 00:16:55 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Reinette Chatre <reinette.chatre@...el.com>
cc:     fenghua.yu@...el.com, tony.luck@...el.com, gavin.hindman@...el.com,
        vikas.shivappa@...ux.intel.com, dave.hansen@...el.com,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V2 06/22] x86/intel_rdt: Create pseudo-locked
 regions

On Mon, 19 Feb 2018, Reinette Chatre wrote:
> On 2/19/2018 12:57 PM, Thomas Gleixner wrote:
> > On Tue, 13 Feb 2018, Reinette Chatre wrote:
> > 
> >> System administrator creates/removes pseudo-locked regions by
> >> creating/removing directories in the pseudo-lock subdirectory of the
> >> resctrl filesystem. Here we add directory creation and removal support.
> >>
> >> A "pseudo-lock region" is introduced, which represents an
> >> instance of a pseudo-locked cache region. During mkdir a new region is
> >> created but since we do not know which cache it belongs to at that time
> >> we maintain a global pointer to it from where it will be moved to the cache
> >> (rdt_domain) it belongs to after initialization. This implies that
> >> we only support one uninitialized pseudo-locked region at a time.
> > 
> > Whats the reason for this restriction? If there are uninitialized
> > directories, so what?
> 
> I was thinking about a problematic scenario where an application
> attempts to create infinite directories. All of these uninitialized
> directories need to be kept track of before they are initialized as
> pseudo-locked regions. It seemed simpler to require that one
> pseudo-locked region is set up at a time.

If the application is allowed to create directories then it can also create
a dozen unused resource control groups. This is not a Joe User operation so
there is no problem.

> >> +/*
> >> + * rdt_pseudo_lock_rmdir - Remove pseudo-lock region
> >> + *
> >> + * LOCKING:
> >> + * Since the pseudo-locked region can be associated with a RDT domain at
> >> + * removal we take both rdtgroup_mutex and rdt_pseudo_lock_mutex to protect
> >> + * the rdt_domain access as well as the pseudo_lock_region access.
> > 
> > Is there a real reason / benefit for having this second mutex?
> 
> Some interactions with the pseudo-locked region are currently done
> without the need for the rdtgroup_mutex. For example, interaction with
> the character device associated with the pseudo-locked region (the
> mmap() call) as well as the debugfs operations.

Well, yes. But none of those operations are hot path so having the double
locking in lots of the other function is just extra complexity for no real
value.

Thanks,

	tglx