linux-0007 — `pktgen`: O(T×D) nested list scan in `__pktgen_NN_threads` and `pktgen_change_name`

Status

PATCHED

Severity

MEDIUM (>20× speedup at T=10, D=100)

Location

net/core/pktgen.c, functions __pktgen_NN_threads() and pktgen_change_name()

Description

The kernel packet generator (pktgen) maintains a two-level data structure:

pktgen_net.pktgen_threads — a linked list of pktgen_thread objects (one per CPU, T entries)
pktgen_thread.if_list — a linked list of pktgen_dev objects per thread (D entries each)

Two functions perform O(T×D) traversal to locate a device:

`__pktgen_NN_threads` (line 2024)

Called from pktgen_lookup_dev(), pktgen_remove_device(), and the proc write path.

list_for_each_entry(t, &pn->pktgen_threads, th_list) {   // O(T)
    pkt_dev = pktgen_find_dev(t, ifname, exact);          // O(D) per thread
    if (pkt_dev) { ... break; }
}

pktgen_find_dev() itself does list_for_each_entry_rcu(p, &t->if_list, list) with strncmp(p->odevname, ifname, len) for each device. Total: O(T×D) per device lookup.

`pktgen_change_name` (line 2082)

Called from the NETDEV_CHANGENAME notifier on every network device rename:

list_for_each_entry(t, &pn->pktgen_threads, th_list) {        // O(T)
    list_for_each_entry(pkt_dev, &t->if_list, list) {          // O(D)
        if (pkt_dev->odev != dev) continue;
        proc_remove(pkt_dev->entry);
        pkt_dev->entry = proc_create_data(dev->name, ...);
        break;
    }
}

Triggered on every interface rename — in environments with many network namespaces and pktgen threads this is O(T×D) per rename event.

Complexity

Slow: O(T × D) per device lookup or rename notification
Fast: O(1) xarray/hashtable lookup keyed by device name or net_device * pointer
At T=10 threads, D=100 devices/thread: 1000 iterations → 1 with the fix

Patch (conceptual — C)

Add an xarray dev_xa field to struct pktgen_net keyed by net_device * pointer. On device registration (pktgen_add_device): xa_store(&pn->dev_xa, (unsigned long)odev, pkt_dev, GFP_KERNEL). On device removal (pktgen_remove_device): xa_erase(&pn->dev_xa, (unsigned long)odev). __pktgen_NN_threads for exact match: xa_load(&pn->dev_xa, (unsigned long)dev) — O(1). pktgen_change_name: xa_load(&pn->dev_xa, (unsigned long)dev) — O(1).

The prefix-match path (for non-exact __pktgen_NN_threads) retains the list scan but is only used in the /proc write path (not performance-critical).

Patch file

See linux-0007-pktgen-thread-dev-xarray.patch

2.5 KiB Raw Blame History Unescape Escape

linux-0007 — pktgen: O(T×D) nested list scan in __pktgen_NN_threads and pktgen_change_name