Commit 1a86a1f7 authored by YiFei Zhu's avatar YiFei Zhu Committed by Paolo Abeni
Browse files

net: Fix rcu_tasks stall in threaded busypoll 



I was debugging a NIC driver when I noticed that when I enable
threaded busypoll, bpftrace hangs when starting up. dmesg showed:

  rcu_tasks_wait_gp: rcu_tasks grace period number 85 (since boot) is 10658 jiffies old.
  rcu_tasks_wait_gp: rcu_tasks grace period number 85 (since boot) is 40793 jiffies old.
  rcu_tasks_wait_gp: rcu_tasks grace period number 85 (since boot) is 131273 jiffies old.
  rcu_tasks_wait_gp: rcu_tasks grace period number 85 (since boot) is 402058 jiffies old.
  INFO: rcu_tasks detected stalls on tasks:
  00000000769f52cd: .N nvcsw: 2/2 holdout: 1 idle_cpu: -1/64
  task:napi/eth2-8265  state:R  running task     stack:0     pid:48300 tgid:48300 ppid:2      task_flags:0x208040 flags:0x00004000
  Call Trace:
   <TASK>
   ? napi_threaded_poll_loop+0x27c/0x2c0
   ? __pfx_napi_threaded_poll+0x10/0x10
   ? napi_threaded_poll+0x26/0x80
   ? kthread+0xfa/0x240
   ? __pfx_kthread+0x10/0x10
   ? ret_from_fork+0x31/0x50
   ? __pfx_kthread+0x10/0x10
   ? ret_from_fork_asm+0x1a/0x30
   </TASK>

The cause is that in threaded busypoll, the main loop is in
napi_threaded_poll rather than napi_threaded_poll_loop, where the
latter rarely iterates more than once within its loop. For
rcu_softirq_qs_periodic inside napi_threaded_poll_loop to report its
qs state, the last_qs must be 100ms behind, and this can't happen
because napi_threaded_poll_loop rarely iterates in threaded busypoll,
and each time napi_threaded_poll_loop is called last_qs is reset to
latest jiffies.

This patch changes so that in threaded busypoll, last_qs is saved
in the outer napi_threaded_poll, and whether busy_poll_last_qs
is NULL indicates whether napi_threaded_poll_loop is called for
busypoll. This way last_qs would not reset to latest jiffies on
each invocation of napi_threaded_poll_loop.

Fixes: c18d4b19 ("net: Extend NAPI threaded polling to allow kthread based busy polling")
Cc: stable@vger.kernel.org
Signed-off-by: default avatarYiFei Zhu <zhuyifei@google.com>
Reviewed-by: default avatarSamiullah Khawaja <skhawaja@google.com>
Link: https://patch.msgid.link/20260227221937.1060857-1-zhuyifei@google.com


Signed-off-by: default avatarPaolo Abeni <pabeni@redhat.com>
parent 6a877ece

net/core/dev.c

+11 −6
Original line number Diff line number Diff line
@@ -7794,11 +7794,12 @@ static int napi_thread_wait(struct napi_struct *napi) 
	return -1;

}



static void napi_threaded_poll_loop(struct napi_struct *napi, bool busy_poll)

static void napi_threaded_poll_loop(struct napi_struct *napi,

				    unsigned long *busy_poll_last_qs)

{

	unsigned long last_qs = busy_poll_last_qs ? *busy_poll_last_qs : jiffies;

	struct bpf_net_context __bpf_net_ctx, *bpf_net_ctx;

	struct softnet_data *sd;

	unsigned long last_qs = jiffies;



	for (;;) {

		bool repoll = false;
@@ -7827,12 +7828,12 @@ static void napi_threaded_poll_loop(struct napi_struct *napi, bool busy_poll) 
		/* When busy poll is enabled, the old packets are not flushed in

		 * napi_complete_done. So flush them here.

		 */

		if (busy_poll)

		if (busy_poll_last_qs)

			gro_flush_normal(&napi->gro, HZ >= 1000);

		local_bh_enable();



		/* Call cond_resched here to avoid watchdog warnings. */

		if (repoll || busy_poll) {

		if (repoll || busy_poll_last_qs) {

			rcu_softirq_qs_periodic(last_qs);

			cond_resched();

		}
@@ -7840,11 +7841,15 @@ static void napi_threaded_poll_loop(struct napi_struct *napi, bool busy_poll) 
		if (!repoll)

			break;

	}



	if (busy_poll_last_qs)

		*busy_poll_last_qs = last_qs;

}



static int napi_threaded_poll(void *data)

{

	struct napi_struct *napi = data;

	unsigned long last_qs = jiffies;

	bool want_busy_poll;

	bool in_busy_poll;

	unsigned long val;
@@ -7862,7 +7867,7 @@ static int napi_threaded_poll(void *data) 
			assign_bit(NAPI_STATE_IN_BUSY_POLL, &napi->state,

				   want_busy_poll);



		napi_threaded_poll_loop(napi, want_busy_poll);

		napi_threaded_poll_loop(napi, want_busy_poll ? &last_qs : NULL);

	}



	return 0;
@@ -13175,7 +13180,7 @@ static void run_backlog_napi(unsigned int cpu) 
{

	struct softnet_data *sd = per_cpu_ptr(&softnet_data, cpu);



	napi_threaded_poll_loop(&sd->backlog, false);

	napi_threaded_poll_loop(&sd->backlog, NULL);

}



static void backlog_napi_setup(unsigned int cpu)