linux-kernel - Re: [PATCH V2 2/5] cpufreq: Replace few CPUFREQ_CONST_LOOPS checks with has

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 27 Jun 2019 10:30:48 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Rafael Wysocki <rjw@...ysocki.net>
Cc:     linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/5] cpufreq: Replace few CPUFREQ_CONST_LOOPS checks
 with has_target()

On 20-06-19, 08:35, Viresh Kumar wrote:
> > CPUFREQ_CONST_LOOPS was introduced in a very old commit from pre-2.6
> > kernel release commit 6a4a93f9c0d5 ("[CPUFREQ] Fix 'out of sync'
> > issue").
> > 
> > Probably the initial idea was to just avoid these checks for set_policy
> > type drivers and then things got changed over the years. And it is very
> > unclear why these checks are there at all.
> > 
> > Replace the CPUFREQ_CONST_LOOPS check with has_target(), which makes
> > more sense now.
> > 
> > cpufreq_notify_transition() is only called for has_target() type driver
> > and not for set_policy type, and the check is simply redundant. Remove
> > it as well.
> > 
> > Also remove () around freq comparison statement as they aren't required
> > and checkpatch also warns for them.
> > 
> > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> > ---
> >  drivers/cpufreq/cpufreq.c | 13 +++++--------
> >  1 file changed, 5 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index 54befd775bd6..41ac701e324f 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -359,12 +359,10 @@ static void cpufreq_notify_transition(struct cpufreq_policy *policy,
> >  		 * which is not equal to what the cpufreq core thinks is
> >  		 * "old frequency".
> >  		 */
> > -		if (!(cpufreq_driver->flags & CPUFREQ_CONST_LOOPS)) {
> > -			if (policy->cur && (policy->cur != freqs->old)) {
> > -				pr_debug("Warning: CPU frequency is %u, cpufreq assumed %u kHz\n",
> > -					 freqs->old, policy->cur);
> > -				freqs->old = policy->cur;
> > -			}
> > +		if (policy->cur && policy->cur != freqs->old) {
> > +			pr_debug("Warning: CPU frequency is %u, cpufreq assumed %u kHz\n",
> > +				 freqs->old, policy->cur);
> > +			freqs->old = policy->cur;
> >  		}
> >  
> >  		srcu_notifier_call_chain(&cpufreq_transition_notifier_list,
> > @@ -1618,8 +1616,7 @@ static unsigned int __cpufreq_get(struct cpufreq_policy *policy)
> >  	if (policy->fast_switch_enabled)
> >  		return ret_freq;
> >  
> > -	if (ret_freq && policy->cur &&
> > -		!(cpufreq_driver->flags & CPUFREQ_CONST_LOOPS)) {
> > +	if (has_target() && ret_freq && policy->cur) {
> >  		/* verify no discrepancy between actual and
> >  					saved value exists */
> >  		if (unlikely(ret_freq != policy->cur)) {

@Rafael: Here are your comments from the IRC exchange we had
yesterday:

> <rafael>:
> 
> so the problem is that, because of the CPUFREQ_CONST_LOOPS check in
> __cpufreq_get(), it almost never does the cpufreq_out_of_sync() thing
> now. Because many drivers set CPUFREQ_CONST_LOOPS most of the time,
> some of them even unconditionally. This patch changes the code that
> runs very rarely into code that runs relatively often.

Right, we will do the frequency verification on has_target() platforms
with CPUFREQ_CONST_LOOPS set after this patch. But why is it the wrong
thing to do ?

What we do here is that we verify that the cached value of current
frequency is same as the real frequency the hardware is running at. It
makes sense to not do this check for setpolicy type drivers as the
cpufreq core isn't always aware of what the driver will end up doing
with the frequency and so no verification.

But for has_target() type drivers, cpufreq core caches the value with
it and it should check it to make sure everything is fine. I don't see
a correlation with CPUFREQ_CONST_LOOPS flag here, that's it. Either we
do this verification or we don't, but there is no reason (as per my
understanding) of skipping it using this flag.

So if you look at the commit I pointed in the history git [1], it does
two things:
- It adds the verification code (which is quite similar today as
  well).
- And it sets the CPUFREQ_CONST_LOOPS flag only for setpolicy drivers,
  rightly so.

The problem happened when we started to use CPUFREQ_CONST_LOOPS for
constant loops-per-jiffy thing as well and many has_target() drivers
started using the same flag and unknowingly skipped the verification
of frequency.

So, I think the current code is doing the wrong thing by skipping the
verification using CPUFREQ_CONST_LOOPS flag.

-- 
viresh

[1] https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/?id=6a4a93f9c0d51b5f4ac1bd3efab53e43584330dd