Re: [PATCH 2/3] tpm: reduce poll sleep time between send() and recv() in tpm_transmit()

From: Mimi Zohar <zohar@linux.vnet.ibm.com>
To: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
	Nayna Jain <nayna@linux.vnet.ibm.com>
Cc: linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-kernel@vger.kernel.org, peterhuewe@gmx.de,
	tpmdd@selhorst.net, jgunthorpe@obsidianresearch.com,
	patrickc@us.ibm.com
Subject: Re: [PATCH 2/3] tpm: reduce poll sleep time between send() and recv() in tpm_transmit()
Date: Mon, 05 Mar 2018 14:07:32 -0500	[thread overview]
Message-ID: <1520276852.10396.351.camel@linux.vnet.ibm.com> (raw)
In-Reply-To: <20180305180144.GE5791@linux.intel.com>

On Mon, 2018-03-05 at 20:01 +0200, Jarkko Sakkinen wrote:
> On Mon, Mar 05, 2018 at 12:56:33PM +0200, Jarkko Sakkinen wrote:
> > On Fri, Mar 02, 2018 at 12:26:35AM +0530, Nayna Jain wrote:
> > > 
> > > 
> > > On 03/01/2018 02:52 PM, Jarkko Sakkinen wrote:
> > > > On Wed, Feb 28, 2018 at 02:18:27PM -0500, Nayna Jain wrote:
> > > > > In tpm_transmit, after send(), the code checks for status in a loop
> > > > Maybe cutting hairs now but please just use the actual function name
> > > > instead of send(). Just makes the commit log more useful asset.
> > > Sure, will do.
> > > > 
> > > > > -		tpm_msleep(TPM_TIMEOUT);
> > > > > +		tpm_msleep(TPM_TIMEOUT_POLL);
> > > > What about just calling schedule()?
> > > I'm not sure what you mean by "schedule()".  Are you suggesting instead of
> > > using usleep_range(),  using something with an even finer grain construct?
> > > 
> > > Thanks & Regards,
> > >      - Nayna
> > 
> > kernel/sched/core.c
> 
> The question I'm trying ask to is: is it better to sleep such a short
> time or just ask scheduler to schedule something else after each
> iteration?

I still don't understand why scheduling some work would be an
improvement.  We still need to loop, testing for the TPM command to
complete.

According to the schedule_hrtimeout_range() function comment,
schedule_hrtimeout_range() is both power and performance friendly.
 What we didn't realize is that the hrtimer *uses* the maximum range
to calculate the sleep time, but *may* return earlier based on the
minimum time.

This patch minimizes the tpm_msleep().  The subsequent patch in this
patch set shows that 1 msec isn't fine enough granularity.  I still
think calling usleep_range() is the right solution, but it needs to be
at a finer granularity than tpm_msleep() provides.

Mimi