linux-kernel - Re: [PATCH 1/8] perf/x86/uncore: Save the unit control address of all units

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Date: Wed, 12 Jun 2024 10:08:13 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: "Liang, Kan" <kan.liang@...ux.intel.com>, peterz@...radead.org, 
	mingo@...nel.org, linux-kernel@...r.kernel.org
Cc: acme@...nel.org, namhyung@...nel.org, irogers@...gle.com,
 eranian@...gle.com,  ak@...ux.intel.com, yunying.sun@...el.com
Subject: Re: [PATCH 1/8] perf/x86/uncore: Save the unit control address of
 all units

On Wed, 2024-06-12 at 10:49 -0400, Liang, Kan wrote:
> 
> On 2024-06-10 6:40 p.m., Tim Chen wrote:
> > On Mon, 2024-06-10 at 13:16 -0700, kan.liang@...ux.intel.com wrote:
> > > From: Kan Liang <kan.liang@...ux.intel.com>
> > > 
> > > The unit control address of some CXL units may be wrongly calculated
> > > under some configuration on a EMR machine.
> > > 
> > > The current implementation only saves the unit control address of the
> > > units from the first die, and the first unit of the rest of dies. Perf
> > > assumed that the units from the other dies have the same offset as the
> > > first die. So the unit control address of the rest of the units can be
> > > calculated. However, the assumption is wrong, especially for the CXL
> > > units.
> > > 
> > > Introduce an RB tree for each uncore type to save the unit control
> > > address and ID information for all the units.
> > > 
> > > Compared with the current implementation, more space is required to save
> > > the information of all units. The extra size should be acceptable.
> > > For example, on EMR, there are 221 units at most. For a 2-socket machine,
> > > the extra space is ~6KB at most.
> > > 
> > > Tested-by: Yunying Sun <yunying.sun@...el.com>
> > > Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
> > > ---
> > >  arch/x86/events/intel/uncore_discovery.c | 79 +++++++++++++++++++++++-
> > >  arch/x86/events/intel/uncore_discovery.h | 10 +++
> > >  2 files changed, 87 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/x86/events/intel/uncore_discovery.c b/arch/x86/events/intel/uncore_discovery.c
> > > index 9a698a92962a..ce520e69a3c1 100644
> > > --- a/arch/x86/events/intel/uncore_discovery.c
> > > +++ b/arch/x86/events/intel/uncore_discovery.c
> > > @@ -93,6 +93,8 @@ add_uncore_discovery_type(struct uncore_unit_discovery *unit)
> > >  	if (!type->box_ctrl_die)
> > >  		goto free_type;
> > >  
> > > +	type->units = RB_ROOT;
> > > +
> > >  	type->access_type = unit->access_type;
> > >  	num_discovered_types[type->access_type]++;
> > >  	type->type = unit->box_type;
> > > @@ -120,10 +122,59 @@ get_uncore_discovery_type(struct uncore_unit_discovery *unit)
> > >  	return add_uncore_discovery_type(unit);
> > >  }
> > >  
> > > +static inline bool unit_less(struct rb_node *a, const struct rb_node *b)
> > > +{
> > > +	struct intel_uncore_discovery_unit *a_node, *b_node;
> > > +
> > > +	a_node = rb_entry(a, struct intel_uncore_discovery_unit, node);
> > > +	b_node = rb_entry(b, struct intel_uncore_discovery_unit, node);
> > > +
> > > +	if (a_node->pmu_idx < b_node->pmu_idx)
> > > +		return true;
> > > +	if (a_node->pmu_idx > b_node->pmu_idx)
> > > +		return false;
> > > +
> > > +	if (a_node->die < b_node->die)
> > > +		return true;
> > > +	if (a_node->die > b_node->die)
> > > +		return false;
> > > +
> > > +	return 0;
> > 
> > Will it be better if the rb_node is sorted by id instead
> > of pmu_idx+die?
> 
> The id and pmu_idx+die can all be used as a key to search the RB tree in
> different places.
> 
> The id is the physical ID of a unit. The search via id is invoked when
> adding a new unit. Perf needs to make sure that the same PMU idx
> (logical id) is assigned to the unit with the same physical ID. Because
> the units with the same physical ID in different dies share the same PMU.
> 
> The pmu_idx+die key is used when setting the cpumask. Please see
> intel_uncore_find_discovery_unit_id() in the patch 2. Perf wants to
> understand on which dies the given PMU is available.
> 
> Since different keys can be used to search the RB tree, I think one of
> them has to traverse the whole tree. At the stage of adding a new unit,
> the tree is not complete yet. It minimizes the impact of the O(N)
> search. So I choose the pmu_idx+die rather than id.
> 
> Also, the driver only does once to build the tree and set the cpumask at
> driver load time. I think the O(N) should be acceptable here.
> 

Thanks for explaining.  I think you saying you are looking up pmu in the tree with id
once during driver load time but needs to look up the tree by die and pmu_idx
more frequently, hence the choice for index.

May be worth mentioning that in a comment.

Tim

> Thanks,
> Kan
> 
> > 
> > Then it will be faster for uncore_find_unit() to run in
> > O(log(N)) instead of O(N).  Right now it looks like we
> > are traversing the whole tree to find the entry with the
> > id.
> > 
> > Tim
> > 
> > > +}
> > > +
> > > +static inline struct intel_uncore_discovery_unit *
> > > +uncore_find_unit(struct rb_root *root, unsigned int id)
> > > +{
> > > +	struct intel_uncore_discovery_unit *unit;
> > > +	struct rb_node *node;
> > > +
> > > +	for (node = rb_first(root); node; node = rb_next(node)) {
> > > +		unit = rb_entry(node, struct intel_uncore_discovery_unit, node);
> > > +		if (unit->id == id)
> > > +			return unit;
> > > +	}
> > > +
> > > +	return NULL;
> > > +}
> > > +
> > 
> >