On 22 Jun 2023, at 0:32, Ilya Maximets wrote:
> Current implementation of meters in the userspace datapath takes
> the meter lock for every packet batch. If more than one thread
> hits the flow with the same meter, they will lock each other.
>
> Replace the critical section with atomic operations to avoid
> interlocking. Meters themselves are RCU-protected, so it's safe
> to access them without holding a lock.
>
> Implementation does the following:
>
> 1. Tries to advance the 'used' timer of the meter with atomic
> compare+exchange if it's smaller than 'now'.
> 2. If the timer change succeeds, atomically update band buckets.
> 3. Atomically update packet statistics for a meter.
> 4. Go over buckets and try to atomically subtract the amount of
> packets or bytes, recording the highest exceeded band.
> 5. Atomically update band statistics and drop packets.
>
> Bucket manipulations are implemented with atomic compare+exchange
> operations with extra checks, because bucket size should never
> exceed the maximum and it should never go below zero.
>
> Packet statistics may be momentarily inconsistent, i.e., number
> of packets and the number of bytes may reflect different sets
> of packets. But it should be eventually consistent. And the
> difference at any given time should be in just few packets.
>
> For the sake of reduced code complexity PKTPS meter tries to push
> packets through the band one by one, even though they all have
> the same weight. This is also more fair if more than one thread
> is passing packets through the same band at the same time.
> Trying to predict the number of packets that can pass may also
> cause extra atomic operations reducing the performance.
>
> This implementation shows similar performance to the previous one,
> but should scale better with more threads hiting the same meter.
This works looks great!! Some small comments below. Did limited testing and
seems to work fine.
Cheers,
Eelco
> Signed-off-by: Ilya Maximets <i.maxim...@ovn.org>
> ---
>
> @Lin Huang, if you can try this change on your setup, that
> would be great.
>
> NEWS | 2 +
> lib/dpif-netdev.c | 250 +++++++++++++++++++++++++---------------------
> 2 files changed, 140 insertions(+), 112 deletions(-)
>
> diff --git a/NEWS b/NEWS
> index 66d5a4ea3..4a2b7dbca 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -37,6 +37,8 @@ Post-v3.1.0
> - SRv6 Tunnel Protocol
> * Added support for userspace datapath (only).
> - Userspace datapath:
> + * Implementation of OpenFlow meters is now lockless allowing for better
> + multi-thread scalability.
> * IP and L4 checksum offload support is now enabled by default for
> interfaces that support it. See the 'status' column in the
> 'interface'
> table to check the status.
> diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
> index abe63412e..2fa556a62 100644
> --- a/lib/dpif-netdev.c
> +++ b/lib/dpif-netdev.c
> @@ -212,21 +212,21 @@ static void dpcls_remove(struct dpcls *, struct
> dpcls_rule *);
> struct dp_meter_band {
> uint32_t rate;
> uint32_t burst_size;
> - uint64_t bucket; /* In 1/1000 packets (for PKTPS), or in bits (for KBPS)
> */
> - uint64_t packet_count;
> - uint64_t byte_count;
> + atomic_uint64_t bucket; /* In 1/1000 packets for PKTPS,
> + * or in bits for KBPS. */
> + atomic_uint64_t packet_count;
> + atomic_uint64_t byte_count;
> };
>
> struct dp_meter {
> struct cmap_node node;
> - struct ovs_mutex lock;
> uint32_t id;
> uint16_t flags;
> uint16_t n_bands;
> uint32_t max_delta_t;
> - uint64_t used;
> - uint64_t packet_count;
> - uint64_t byte_count;
> + atomic_uint64_t used; /* Time of a last use in milliseconds. */
> + atomic_uint64_t packet_count;
> + atomic_uint64_t byte_count;
> struct dp_meter_band bands[];
> };
>
> @@ -7165,22 +7165,56 @@ dpif_netdev_meter_get_features(const struct dpif *
> dpif OVS_UNUSED,
> features->max_color = 0;
> }
>
> +/* Tries to atomically add 'n' to 'value' in terms of saturation arithmetic,
> + * i.e., if the result will be larger than 'max_value', will store
> 'max_value'
> + * instead. */
> +static void
> +atomic_sat_add(atomic_uint64_t *value, uint64_t n, uint64_t max_value)
> +{
> + uint64_t current, new_value;
> +
> + atomic_read_relaxed(value, ¤t);
> + do {
> + new_value = current + n;
> + new_value = MIN(new_value, max_value);
> + } while (!atomic_compare_exchange_weak_relaxed(value, ¤t,
> + new_value));
> +}
> +
> +/* Tries to atomically subtract 'n' from 'value'. Does not perform the
> + * operation and returns 'false' if the result will be less than 'min_value'.
> + * Otherwise, stores the result and returns 'true'. */
> +static bool
> +atomic_bound_sub(atomic_uint64_t *value, uint64_t n, uint64_t min_value)
> +{
> + uint64_t current;
> +
> + atomic_read_relaxed(value, ¤t);
> + do {
> + if (current < min_value + n) {
> + return false;
> + }
> + } while (!atomic_compare_exchange_weak_relaxed(value, ¤t,
> + current - n));
> + return true;
> +}
> +
> /* Applies the meter identified by 'meter_id' to 'packets_'. Packets
> * that exceed a band are dropped in-place. */
> static void
> dp_netdev_run_meter(struct dp_netdev *dp, struct dp_packet_batch *packets_,
> - uint32_t meter_id, long long int now)
> + uint32_t meter_id, long long int now_ms)
> {
> - struct dp_meter *meter;
> - struct dp_meter_band *band;
> - struct dp_packet *packet;
> - long long int long_delta_t; /* msec */
> - uint32_t delta_t; /* msec */
> const size_t cnt = dp_packet_batch_size(packets_);
> - uint32_t bytes, volume;
> - int exceeded_band[NETDEV_MAX_BURST];
> uint32_t exceeded_rate[NETDEV_MAX_BURST];
> - int exceeded_pkt = cnt; /* First packet that exceeded a band rate. */
> + uint32_t exceeded_band[NETDEV_MAX_BURST];
> + uint64_t bytes, volume, meter_used, old;
> + uint64_t band_packets[MAX_BANDS];
> + uint64_t band_bytes[MAX_BANDS];
> + struct dp_meter_band *band;
> + struct dp_packet *packet;
> + struct dp_meter *meter;
> + bool exceeded = false;
>
> if (meter_id >= MAX_METERS) {
> return;
> @@ -7196,116 +7230,102 @@ dp_netdev_run_meter(struct dp_netdev *dp, struct
> dp_packet_batch *packets_,
> /* Initialize as zeroes. */
> memset(exceeded_rate, 0, cnt * sizeof *exceeded_rate);
>
> - ovs_mutex_lock(&meter->lock);
> - /* All packets will hit the meter at the same time. */
> - long_delta_t = now / 1000 - meter->used / 1000; /* msec */
> + atomic_read_relaxed(&meter->used, &meter_used);
> + do {
> + if (meter_used >= now_ms) {
> + /* The '>' condition means that we have several threads hitting
> the
> + * same meter, and the other one already advanced the time. */
> + meter_used = now_ms;
> + break;
> + }
> + } while (!atomic_compare_exchange_weak_relaxed(&meter->used,
> + &meter_used, now_ms));
>
> - if (long_delta_t < 0) {
> - /* This condition means that we have several threads fighting for a
> - meter lock, and the one who received the packets a bit later wins.
> - Assuming that all racing threads received packets at the same time
> - to avoid overflow. */
> - long_delta_t = 0;
> - }
> + /* Refill all buckets right away, since other threads may use them. */
> + if (meter_used < now_ms) {
> + /* All packets will hit the meter at the same time. */
> + uint64_t delta_t = now_ms - meter_used;
> +
> + /* Make sure delta_t will not be too large, so that bucket will not
> + * wrap around below. */
> + delta_t = MIN(delta_t, meter->max_delta_t);
>
> - /* Make sure delta_t will not be too large, so that bucket will not
> - * wrap around below. */
> - delta_t = (long_delta_t > (long long int)meter->max_delta_t)
> - ? meter->max_delta_t : (uint32_t)long_delta_t;
> + for (int m = 0; m < meter->n_bands; m++) {
> + band = &meter->bands[m];
> + /* Update band's bucket. We can't just use atomic add here,
> + * because we should never add above the max capacity. */
> + atomic_sat_add(&band->bucket, delta_t * band->rate,
> + band->burst_size * 1000ULL);
> + }
> + }
>
> /* Update meter stats. */
> - meter->used = now;
> - meter->packet_count += cnt;
> + atomic_add_relaxed(&meter->packet_count, cnt, &old);
> bytes = 0;
> DP_PACKET_BATCH_FOR_EACH (i, packet, packets_) {
> bytes += dp_packet_size(packet);
> }
> - meter->byte_count += bytes;
> + atomic_add_relaxed(&meter->byte_count, bytes, &old);
>
> /* Meters can operate in terms of packets per second or kilobits per
> * second. */
> if (meter->flags & OFPMF13_PKTPS) {
> - /* Rate in packets/second, bucket 1/1000 packets. */
> - /* msec * packets/sec = 1/1000 packets. */
> + /* Rate in packets/second, bucket 1/1000 packets.
> + * msec * packets/sec = 1/1000 packets. */
> volume = cnt * 1000; /* Take 'cnt' packets from the bucket. */
> } else {
> - /* Rate in kbps, bucket in bits. */
> - /* msec * kbps = bits */
> + /* Rate in kbps, bucket in bits.
> + * msec * kbps = bits */
> volume = bytes * 8;
> }
>
> - /* Update all bands and find the one hit with the highest rate for each
> - * packet (if any). */
> - for (int m = 0; m < meter->n_bands; ++m) {
> - uint64_t max_bucket_size;
> -
> + /* Find the band hit with the highest rate for each packet (if any). */
> + for (int m = 0; m < meter->n_bands; m++) {
> band = &meter->bands[m];
> - max_bucket_size = band->burst_size * 1000ULL;
> - /* Update band's bucket. */
> - band->bucket += (uint64_t) delta_t * band->rate;
> - if (band->bucket > max_bucket_size) {
> - band->bucket = max_bucket_size;
> - }
>
> /* Drain the bucket for all the packets, if possible. */
> - if (band->bucket >= volume) {
> - band->bucket -= volume;
> - } else {
> - int band_exceeded_pkt;
> -
> - /* Band limit hit, must process packet-by-packet. */
> - if (meter->flags & OFPMF13_PKTPS) {
> - band_exceeded_pkt = band->bucket / 1000;
> - band->bucket %= 1000; /* Remainder stays in bucket. */
> -
> - /* Update the exceeding band for each exceeding packet.
> - * (Only one band will be fired by a packet, and that
> - * can be different for each packet.) */
> - for (int i = band_exceeded_pkt; i < cnt; i++) {
> - if (band->rate > exceeded_rate[i]) {
> - exceeded_rate[i] = band->rate;
> - exceeded_band[i] = m;
> - }
> - }
> - } else {
> - /* Packet sizes differ, must process one-by-one. */
> - band_exceeded_pkt = cnt;
> - DP_PACKET_BATCH_FOR_EACH (i, packet, packets_) {
> - uint32_t bits = dp_packet_size(packet) * 8;
> -
> - if (band->bucket >= bits) {
> - band->bucket -= bits;
> - } else {
> - if (i < band_exceeded_pkt) {
> - band_exceeded_pkt = i;
> - }
> - /* Update the exceeding band for the exceeding
> packet.
> - * (Only one band will be fired by a packet, and that
> - * can be different for each packet.) */
> - if (band->rate > exceeded_rate[i]) {
> - exceeded_rate[i] = band->rate;
> - exceeded_band[i] = m;
> - }
> - }
> + if (atomic_bound_sub(&band->bucket, volume, 0)) {
> + continue;
> + }
> +
> + /* Band limit hit, must process packet-by-packet. */
> + DP_PACKET_BATCH_FOR_EACH (i, packet, packets_) {
> + uint64_t packet_volume = (meter->flags & OFPMF13_PKTPS)
> + ? 1000 : (dp_packet_size(packet) * 8);
> +
> + if (!atomic_bound_sub(&band->bucket, packet_volume, 0)) {
> + /* Update the exceeding band for the exceeding packet.
> + * Only one band will be fired by a packet, and that can
> + * be different for each packet. */
> + if (band->rate > exceeded_rate[i]) {
> + exceeded_rate[i] = band->rate;
> + exceeded_band[i] = m;
> + exceeded = true;
> }
> }
> - /* Remember the first exceeding packet. */
> - if (exceeded_pkt > band_exceeded_pkt) {
> - exceeded_pkt = band_exceeded_pkt;
> - }
> }
> }
>
> + /* No need to iterate over packets if there are no drops. */
> + if (!exceeded) {
> + return;
> + }
> +
> /* Fire the highest rate band exceeded by each packet, and drop
> * packets if needed. */
> +
Don’t think we need this newline.
> + memset(band_packets, 0, sizeof band_packets);
> + memset(band_bytes, 0, sizeof band_bytes);
Are these extra spaces for alignment on purpose?
> +
> size_t j;
> DP_PACKET_BATCH_REFILL_FOR_EACH (j, cnt, packet, packets_) {
> - if (exceeded_band[j] >= 0) {
> + uint32_t m = exceeded_band[j];
> +
> + if (m != UINT32_MAX) {
> /* Meter drop packet. */
> - band = &meter->bands[exceeded_band[j]];
> - band->packet_count += 1;
> - band->byte_count += dp_packet_size(packet);
> - COVERAGE_INC(datapath_drop_meter);
> + band = &meter->bands[m];
> + band_packets[m]++;
> + band_bytes[m] += dp_packet_size(packet);
This code now looks like this (the diff is a mess to comment on):
if (m != UINT32_MAX) {
/* Meter drop packet. */
band = &meter->bands[m];
! ^^^ This line can be removed as band is not used.
band_packets[m]++;
band_bytes[m] += dp_packet_size(packet);
dp_packet_delete(packet);
} else {
/* Meter accepts packet. */
dp_packet_batch_refill(packets_, packet, j);
}
}
> dp_packet_delete(packet);
> } else {
> /* Meter accepts packet. */
> @@ -7313,7 +7333,15 @@ dp_netdev_run_meter(struct dp_netdev *dp, struct
> dp_packet_batch *packets_,
> }
> }
>
> - ovs_mutex_unlock(&meter->lock);
> + for (int m = 0; m < meter->n_bands; m++) {
> + if (!band_packets[m]) {
> + continue;
> + }
> + band = &meter->bands[m];
> + atomic_add_relaxed(&band->packet_count, band_packets[m], &old);
> + atomic_add_relaxed(&band->byte_count, band_bytes[m], &old);
Are these extra spaces for alignment on purpose?
> + COVERAGE_ADD(datapath_drop_meter, band_packets[m]);
> + }
> }
>
> /* Meter set/get/del processing is still single-threaded. */
> @@ -7354,13 +7382,13 @@ dpif_netdev_meter_set(struct dpif *dpif,
> ofproto_meter_id meter_id,
> meter->flags = config->flags;
> meter->n_bands = config->n_bands;
> meter->max_delta_t = 0;
> - meter->used = time_usec();
> meter->id = mid;
> - ovs_mutex_init_adaptive(&meter->lock);
> + atomic_init(&meter->used, time_msec());
>
> /* set up bands */
> for (i = 0; i < config->n_bands; ++i) {
> uint32_t band_max_delta_t;
> + uint64_t bucket_size;
>
> /* Set burst size to a workable value if none specified. */
> if (config->bands[i].burst_size == 0) {
> @@ -7370,11 +7398,11 @@ dpif_netdev_meter_set(struct dpif *dpif,
> ofproto_meter_id meter_id,
> meter->bands[i].rate = config->bands[i].rate;
> meter->bands[i].burst_size = config->bands[i].burst_size;
> /* Start with a full bucket. */
> - meter->bands[i].bucket = meter->bands[i].burst_size * 1000ULL;
> + bucket_size = meter->bands[i].burst_size * 1000ULL;
> + atomic_init(&meter->bands[i].bucket, bucket_size);
>
> /* Figure out max delta_t that is enough to fill any bucket. */
> - band_max_delta_t
> - = meter->bands[i].bucket / meter->bands[i].rate;
> + band_max_delta_t = bucket_size / meter->bands[i].rate;
> if (band_max_delta_t > meter->max_delta_t) {
> meter->max_delta_t = band_max_delta_t;
> }
> @@ -7397,7 +7425,7 @@ dpif_netdev_meter_get(const struct dpif *dpif,
> {
> struct dp_netdev *dp = get_dp_netdev(dpif);
> uint32_t meter_id = meter_id_.uint32;
> - const struct dp_meter *meter;
> + struct dp_meter *meter;
>
> if (meter_id >= MAX_METERS) {
> return EFBIG;
> @@ -7411,17 +7439,15 @@ dpif_netdev_meter_get(const struct dpif *dpif,
> if (stats) {
> int i = 0;
>
> - ovs_mutex_lock(&meter->lock);
> -
> - stats->packet_in_count = meter->packet_count;
> - stats->byte_in_count = meter->byte_count;
> + atomic_read_relaxed(&meter->packet_count, &stats->packet_in_count);
> + atomic_read_relaxed(&meter->byte_count, &stats->byte_in_count);
>
> for (i = 0; i < n_bands && i < meter->n_bands; ++i) {
> - stats->bands[i].packet_count = meter->bands[i].packet_count;
> - stats->bands[i].byte_count = meter->bands[i].byte_count;
> + atomic_read_relaxed(&meter->bands[i].packet_count,
> + &stats->bands[i].packet_count);
> + atomic_read_relaxed(&meter->bands[i].byte_count,
> + &stats->bands[i].byte_count);
> }
> -
> - ovs_mutex_unlock(&meter->lock);
> stats->n_bands = i;
> }
>
> @@ -9173,7 +9199,7 @@ dp_execute_cb(void *aux_, struct dp_packet_batch
> *packets_,
>
> case OVS_ACTION_ATTR_METER:
> dp_netdev_run_meter(pmd->dp, packets_, nl_attr_get_u32(a),
> - pmd->ctx.now);
> + pmd->ctx.now / 1000);
> break;
>
> case OVS_ACTION_ATTR_PUSH_VLAN:
> --
> 2.40.1
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