hot busy loop inside ath11k

[Yury Vostrikov]

Hi Kalle,

It seems there is problem with busy wait inside
ath11k_debugfs_fw_stats_request. I have a laptop with QCNFA765 WiFi
controller. It is running vanilla v6.11.4 with the following fw:

> [    3.934078] ath11k_pci 0000:01:00.0: wcn6855 hw2.1
> [    4.801624] ath11k_pci 0000:01:00.0: chip_id 0x12 chip_family 0xb board_id 0xff soc_id 0x400c1211
> [    4.802469] ath11k_pci 0000:01:00.0: fw_version 0x11088c35 fw_build_timestamp 2024-04-17 08:34 fw_build_id WLAN.HSP.1.1-03125-QCAHSPSWPL_V1_V2_SILICONZ_LITE-3.6510.41

Sometimes after device wakes up, the system becomes
sluggish and burns CPU. A assume it is because of a firmware bug, but driver
should not waste CPU regardless.

According to perf, the most time is spent in busy waitin inside  ath11k_debugfs_fw_stats_request:

94.60%     0.00%  i3status         [kernel.kallsyms]                 [k] do_syscall_64
        |
         --94.60%--do_syscall_64
                   |
                    --94.55%--__sys_sendmsg
                              ___sys_sendmsg
                              ____sys_sendmsg
                              netlink_sendmsg
                              netlink_unicast
                              genl_rcv
                              netlink_rcv_skb
                              genl_rcv_msg
                              |
                               --94.55%--genl_family_rcv_msg_dumpit
                                         __netlink_dump_start
                                         netlink_dump
                                         genl_dumpit
                                         nl80211_dump_station
                                         |
                                          --94.55%--ieee80211_dump_station
                                                    sta_set_sinfo
                                                    |
                                                     --94.55%--ath11k_mac_op_sta_statistics
                                                               ath11k_debugfs_get_fw_stats
                                                               |
                                                                --94.55%--ath11k_debugfs_fw_stats_request
                                                                          |
                                                                          |--41.73%--_raw_spin_lock_bh
                                                                          |
                                                                          |--22.74%--__local_bh_enable_ip
                                                                          |
                                                                          |--9.22%--_raw_spin_unlock_bh
                                                                          |
                                                                           --6.66%--srso_alias_safe_ret

If I'm reading the code correctly, then ath11k_debugfs_fw_stats_request has 3 second timeout:

> timeout = jiffies + msecs_to_jiffies(3 * 1000);

however, it only waits for 1 second:

> time_left = wait_for_completion_timeout(&ar->fw_stats_complete, 1 * HZ);

the rest (2 seconds) is spent inside busy loop

>	for (;;) {
>		if (time_after(jiffies, timeout))
>			break;
>
>		spin_lock_bh(&ar->data_lock);
>		if (ar->fw_stats_done) {
>			spin_unlock_bh(&ar->data_lock);
>			break;
>		}
>		spin_unlock_bh(&ar->data_lock);
>	}


spinning  for 2 seconds seems excessive to me. What do you think?

Also, if you happen to know how & where to report firmware bugs, I'd
appreciate any pointers.

thanks