linux-kernel - Re: [PATCH 14/21] x86/intel_rdt/cqm: Add mon

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Date:   Tue, 11 Jul 2017 14:37:56 -0700
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Shivappa Vikas <vikas.shivappa@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org, hpa@...or.com,
        peterz@...radead.org, ravi.v.shankar@...el.com,
        fenghua.yu@...el.com, andi.kleen@...el.com
Subject: Re: [PATCH 14/21] x86/intel_rdt/cqm: Add mon_data

On Tue, Jul 11, 2017 at 02:17:47PM -0700, Shivappa Vikas wrote:
> 
> 
> On Thu, 6 Jul 2017, Thomas Gleixner wrote:
> 
> > On Thu, 6 Jul 2017, Shivappa Vikas wrote:
> > > On Sun, 2 Jul 2017, Thomas Gleixner wrote:
> > > > > +static bool __mon_event_count(u32 rmid, struct rmid_read *rr)
> > > > > +{
> > > > > +	u64 tval;
> > > > > +
> > > > > +	tval = __rmid_read(rmid, rr->evtid);
> > > > > +	if (tval & (RMID_VAL_ERROR | RMID_VAL_UNAVAIL)) {
> > > > > +		rr->val = tval;
> > > > > +		return false;
> > > > > +	}
> > > > > +	switch (rr->evtid) {
> > > > > +	case QOS_L3_OCCUP_EVENT_ID:
> > > > > +		rr->val += tval;
> > > > > +		return true;
> > > > > +	default:
> > > > > +		return false;
> > > > 
> > > > I have no idea what that return code means.
> > > 
> > > false for the invalid event id and all errors for __rmid_read. (IOW all errors
> > > for __mon_event-read)
> > 
> > Sure, but why bool? What's wrong with proper error return codes, so issues
> > can be distinguished and potentially propagated in the callchain?
> 
> Ok, The error is propagated wih the rr->val actually. is this better?
> 
> Hardware throws the RMID_VAL_ERROR (bit 63) when an invalid RMID or
> event is written to event select - this case seems similar.
> 
> 	default:
> 		rr->val = RMID_VAL_ERROR;
> 		return -EINVAL;
> 	}

I'll take the blame for not documenting this better.  What's going
on here is that we are calculating the sum of some list of RMIDs
(for the case where we read a mon_data/*/* file for a CTRL_MON group
that has some MON subgroups ... when reading a from a MON group there
is only one RMID).

Now we might get an error reading one of those (either or both of
RMID_VAL_ERROR and RMID_VAL_UNAVAIL bits set). In which case we can't
compute the sum, and there is no point in reading any more RMIDs.

So the return of this function is:

true: I read this RMID OK and added it to rr->val

false: I got an error. Give up. The error type is in the high bits of rr->val


-Tony