Re: [PATCH v5 net-next 3/3] net: mscc: ocelot: use bulk reads for stats

[Colin Foster]

Hi Vladimir,

On Tue, Feb 08, 2022 at 03:03:04PM +0000, Vladimir Oltean wrote:
> On Mon, Feb 07, 2022 at 08:46:44PM -0800, Colin Foster wrote:
> > Create and utilize bulk regmap reads instead of single access for gathering
> > stats. The background reading of statistics happens frequently, and over
> > a few contiguous memory regions.
> > 
> > High speed PCIe buses and MMIO access will probably see negligible
> > performance increase. Lower speed buses like SPI and I2C could see
> > significant performance increase, since the bus configuration and register
> > access times account for a large percentage of data transfer time.
> > 
> > Signed-off-by: Colin Foster <colin.foster@xxxxxxxxxxxxxxxx>
> > ---
> >  drivers/net/ethernet/mscc/ocelot.c | 78 +++++++++++++++++++++++++-----
> >  include/soc/mscc/ocelot.h          |  8 +++
> >  2 files changed, 73 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/mscc/ocelot.c b/drivers/net/ethernet/mscc/ocelot.c
> > index 455293aa6343..5efb1f3a1410 100644
> > --- a/drivers/net/ethernet/mscc/ocelot.c
> > +++ b/drivers/net/ethernet/mscc/ocelot.c
> > @@ -1737,32 +1737,41 @@ void ocelot_get_strings(struct ocelot *ocelot, int port, u32 sset, u8 *data)
> >  }
> >  EXPORT_SYMBOL(ocelot_get_strings);
> >  
> > -static void ocelot_update_stats(struct ocelot *ocelot)
> > +static int ocelot_update_stats(struct ocelot *ocelot)
> >  {
> > -	int i, j;
> > +	struct ocelot_stats_region *region;
> > +	int i, j, err = 0;
> >  
> >  	mutex_lock(&ocelot->stats_lock);
> >  
> >  	for (i = 0; i < ocelot->num_phys_ports; i++) {
> > +		unsigned int idx = 0;
> > +
> 
> This is a bug which causes ocelot->stats to be overwritten with the
> statistics of port 0, for all ports. Either move the variable
> declaration and initialization with 0 in the larger scope (outside the
> "for" loop), or initialize idx with i * ocelot->num_stats.

I see that now. It is confusing and I'll clear it up. I never caught
this because I'm testing in a setup where port 0 is the CPU port, so I
can't get ethtool stats. Thanks!

> 
> >  		/* Configure the port to read the stats from */
> >  		ocelot_write(ocelot, SYS_STAT_CFG_STAT_VIEW(i), SYS_STAT_CFG);
> >  
> > -		for (j = 0; j < ocelot->num_stats; j++) {
> > -			u32 val;
> > -			unsigned int idx = i * ocelot->num_stats + j;
> > +		list_for_each_entry(region, &ocelot->stats_regions, node) {
> > +			err = ocelot_bulk_read_rix(ocelot, SYS_COUNT_RX_OCTETS,
> > +						   region->offset, region->buf,
> > +						   region->count);
> > +			if (err)
> > +				goto out;
> >  
> > -			val = ocelot_read_rix(ocelot, SYS_COUNT_RX_OCTETS,
> > -					      ocelot->stats_layout[j].offset);
> > +			for (j = 0; j < region->count; j++) {
> > +				if (region->buf[j] < (ocelot->stats[idx + j] & U32_MAX))
> > +					ocelot->stats[idx + j] += (u64)1 << 32;
> 
> I'd prefer if you reduce the apparent complexity of this logic by
> creating some temporary variables:
> 
> 	u64 *stat = &ocelot->stats[idx + j];
> 	u64 val = region->buf[j];

Can do. Thanks for the suggestion.

> 
> >  
> > -			if (val < (ocelot->stats[idx] & U32_MAX))
> > -				ocelot->stats[idx] += (u64)1 << 32;
> > +				ocelot->stats[idx + j] = (ocelot->stats[idx + j] &
> > +							~(u64)U32_MAX) + region->buf[j];
> > +			}
> >  
> > -			ocelot->stats[idx] = (ocelot->stats[idx] &
> > -					      ~(u64)U32_MAX) + val;
> > +			idx += region->count;
> >  		}
> >  	}
> >  
> > +out:
> >  	mutex_unlock(&ocelot->stats_lock);
> > +	return err;
> >  }
> >  
> >  static void ocelot_check_stats_work(struct work_struct *work)
> > @@ -1779,10 +1788,11 @@ static void ocelot_check_stats_work(struct work_struct *work)
> >  
> >  void ocelot_get_ethtool_stats(struct ocelot *ocelot, int port, u64 *data)
> >  {
> > -	int i;
> > +	int i, err;
> >  
> >  	/* check and update now */
> > -	ocelot_update_stats(ocelot);
> > +	err = ocelot_update_stats(ocelot);
> 
> Please, as a separate change, introduce a function that reads the
> statistics for a single port, and make ethtool call that and not the
> entire port array, it's pointless.
> 
> > +	WARN_ONCE(err, "Error %d updating ethtool stats\n", err);
> >  
> >  	/* Copy all counters */
> >  	for (i = 0; i < ocelot->num_stats; i++)
> 
> and here, in the unseen part of the context, lies:
> 
> 	/* Copy all counters */
> 	for (i = 0; i < ocelot->num_stats; i++)
> 		*data++ = ocelot->stats[port * ocelot->num_stats + i];
> 
> I think this is buggy, because this is a reader of ocelot->stats which
> is not protected by ocelot->stats_lock (it was taken and dropped by
> ocelot_update_stats). But a second ocelot_update_stats() can run
> concurrently with ethtool and ruin the day, modifying the array at the
> same time as it's being read out.
> 
> The new function that you introduce, for reading the stats of a single
> port, should require that ocelot->stats_lock is already held, and you
> should hold it from top-level (ocelot_get_ethtool_stats).

I'll add this fix as a separate patch. Thanks as always for the
feedback!

> 
> > @@ -1799,6 +1809,41 @@ int ocelot_get_sset_count(struct ocelot *ocelot, int port, int sset)
> >  }
> >  EXPORT_SYMBOL(ocelot_get_sset_count);
> >  
> > +static int ocelot_prepare_stats_regions(struct ocelot *ocelot)
> > +{
> > +	struct ocelot_stats_region *region = NULL;
> > +	unsigned int last;
> > +	int i;
> > +
> > +	INIT_LIST_HEAD(&ocelot->stats_regions);
> > +
> > +	for (i = 0; i < ocelot->num_stats; i++) {
> > +		if (region && ocelot->stats_layout[i].offset == last + 1) {
> > +			region->count++;
> > +		} else {
> > +			region = devm_kzalloc(ocelot->dev, sizeof(*region),
> > +					      GFP_KERNEL);
> > +			if (!region)
> > +				return -ENOMEM;
> > +
> > +			region->offset = ocelot->stats_layout[i].offset;
> > +			region->count = 1;
> > +			list_add_tail(&region->node, &ocelot->stats_regions);
> > +		}
> > +
> > +		last = ocelot->stats_layout[i].offset;
> > +	}
> > +
> > +	list_for_each_entry(region, &ocelot->stats_regions, node) {
> > +		region->buf = devm_kcalloc(ocelot->dev, region->count,
> > +					   sizeof(*region->buf), GFP_KERNEL);
> > +		if (!region->buf)
> > +			return -ENOMEM;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> >  int ocelot_get_ts_info(struct ocelot *ocelot, int port,
> >  		       struct ethtool_ts_info *info)
> >  {
> > @@ -2799,6 +2844,13 @@ int ocelot_init(struct ocelot *ocelot)
> >  				 ANA_CPUQ_8021_CFG_CPUQ_BPDU_VAL(6),
> >  				 ANA_CPUQ_8021_CFG, i);
> >  
> > +	ret = ocelot_prepare_stats_regions(ocelot);
> > +	if (ret) {
> > +		destroy_workqueue(ocelot->stats_queue);
> > +		destroy_workqueue(ocelot->owq);
> > +		return ret;
> > +	}
> > +
> >  	INIT_DELAYED_WORK(&ocelot->stats_work, ocelot_check_stats_work);
> >  	queue_delayed_work(ocelot->stats_queue, &ocelot->stats_work,
> >  			   OCELOT_STATS_CHECK_DELAY);
> > diff --git a/include/soc/mscc/ocelot.h b/include/soc/mscc/ocelot.h
> > index 312b72558659..d3291a5f7e88 100644
> > --- a/include/soc/mscc/ocelot.h
> > +++ b/include/soc/mscc/ocelot.h
> > @@ -542,6 +542,13 @@ struct ocelot_stat_layout {
> >  	char name[ETH_GSTRING_LEN];
> >  };
> >  
> > +struct ocelot_stats_region {
> > +	struct list_head node;
> > +	u32 offset;
> > +	int count;
> > +	u32 *buf;
> > +};
> > +
> >  enum ocelot_tag_prefix {
> >  	OCELOT_TAG_PREFIX_DISABLED	= 0,
> >  	OCELOT_TAG_PREFIX_NONE,
> > @@ -673,6 +680,7 @@ struct ocelot {
> >  	struct regmap_field		*regfields[REGFIELD_MAX];
> >  	const u32 *const		*map;
> >  	const struct ocelot_stat_layout	*stats_layout;
> > +	struct list_head		stats_regions;
> >  	unsigned int			num_stats;
> >  
> >  	u32				pool_size[OCELOT_SB_NUM][OCELOT_SB_POOL_NUM];
> > -- 
> > 2.25.1
> >