linux-kernel - Re: [PATCH] x86/resctrl: Fix event counts regression in reused RMIDs

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Date:   Thu, 8 Dec 2022 10:30:54 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Peter Newman <peternewman@...gle.com>
CC:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        James Morse <james.morse@....com>,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        <linux-kernel@...r.kernel.org>, <eranian@...gle.com>,
        Babu Moger <Babu.Moger@....com>
Subject: Re: [PATCH] x86/resctrl: Fix event counts regression in reused RMIDs

Hi Peter,

On 12/8/2022 2:04 AM, Peter Newman wrote:
> Hi Reinette,
> 
> On Wed, Dec 7, 2022 at 8:48 PM Reinette Chatre
> <reinette.chatre@...el.com> wrote:
>>
>> To get back to the original behavior before the refactoring it also seems
>> that __mon_event_count() needs to return right after calling
>> resctrl_arch_reset_rmid(). The only caller with rr->first set is when
>> the mon directory is created and the returned values are not used,
>> it is just run to get prev_msr set. This also avoids unnecessarily reading
>> the counters twice.
>>
>> So, how about:
>>
>> static int __mon_event_count(u32 rmid, struct rmid_read *rr)
>> {
>>
>> ...
>> if (rr->first) {
>> resctrl_arch_reset_rmid(rr->r, rr->d, rmid, rr->evtid);
>> return 0;
>> }
>> ...
>>
>> }
> 
> Avoiding the double-read sounds good, but...
> 
>>
>> Also ... there appears to be a leftover related snippet in __mon_event_count()
>> that does not belong anymore and may still cause incorrect behavior:
>>
>> static int __mon_event_count(u32 rmid, struct rmid_read *rr)
>> {
>> ...
>> if (rr->first) {
>> memset(m, 0, sizeof(struct mbm_state));
>> return 0;
>> }
>> ...
>> }
> 
> I'm less sure about removing (or skipping) this. mbm_state::mbm_local
> still seems to be used by the mba_sc code. That might be why James
> left this code in.
> 
> I was sort of confused about the new role of mbm_state following the
> refactoring when reviewing Babu's change. (which reminds me that I
> should have CC'ed him on this patch)


I think this can be cleaned up to make the code more clear. Notice the
duplication of  following snippet in __mon_event_count():
	rr->val += tval;
	return 0;

I do not see any need to check the event id before doing the above. That
leaves the bulk of the switch just needed for the rr->first handling that
can be moved to resctrl_arch_reset_rmid().

Something like:

void resctrl_arch_reset_rmid(struct rdt_resource *r, struct rdt_domain *d, ...
{
	...
	struct arch_mbm_state *am;
	struct mbm_state *m;
	u64 val = 0;
	int ret;
	
	m = get_mbm_state(d, rmid, eventid); /* get_mbm_state() to be created */
	if (m)
		memset(m, 0, sizeof(*m));	

	am = get_arch_mbm_state(hw_dom, rmid, eventid);
	if (am) {
		memset(am, 0, sizeof(*am));	
		/* Record any initial, non-zero count value. */
		ret = __rmid_read(rmid, eventid, &val);
		if (!ret)
			am->prev_msr = val;
	}

}

Having this would be helpful as reference to Babu's usage. 

Also please note that I changed the __rmid_read(). There is no need
to require each __rmid_read() caller to test MSR bits for validity, that
can be contained within __rmid_read().

Something like below remains:

static int __mon_event_count(u32 rmid, struct rmid_read *rr)
{

	...

	if (rr->first) {
		resctrl_arch_reset_rmid(rr->r, rr->d, rmid, rr->evtid);
		return 0;
	}

	rr->err = resctrl_arch_rmid_read(rr->r, rr->d, rmid, rr->evtid, &tval);
	if (rr->err)
		return rr->err;

	rr->val += tval;
	return 0;

}

What do you think? 

Reinette