Re: [RFC 06/10] Reclaim memory from blocked kernel stacks

[Steven Rostedt]

On Fri, 28 Aug 2026 15:08:36 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> If you want to see how big your stack is and where its used, you can run
> the stack tracer:
> 
>  # trace-cmd stack --start
> 
> [wait]
> 
>  # trace-cmd stack
> (stack tracer running)
>         Depth    Size   Location    (41 entries)
>         -----    ----   --------
>   0)     5032      48   choose_idle_cpu+0x5/0x100
>   1)     4984     296   select_task_rq_fair+0xa84/0x2b40
>   2)     4688     104   try_to_wake_up+0x168/0x7e0
>   3)     4584      16   task_work_add+0xd7/0xf0
>   4)     4568      16   io_req_normal_work_add+0x75/0xb0
>   5)     4552      40   io_poll_wake+0x10f/0x160
>   6)     4512      64   __wake_up_common+0x72/0xa0
>   7)     4448      40   __wake_up_sync_key+0x43/0x60
>   8)     4408      24   sock_def_readable+0x46/0xe0
>   9)     4384      80   tun_net_xmit+0x240/0x550 [tun]
>  10)     4304      80   dev_hard_start_xmit+0x63/0x1e0
>  11)     4224     256   __dev_queue_xmit+0x880/0x10b0
>  12)     3968      24   br_dev_queue_push_xmit+0x62/0xf0 [bridge]
>  13)     3944      80   br_dev_xmit+0x15e/0x4a0 [bridge]
>  14)     3864      80   dev_hard_start_xmit+0x63/0x1e0
>  15)     3784     256   __dev_queue_xmit+0x880/0x10b0
>  16)     3528      72   ip_finish_output2+0x2ce/0x650
>  17)     3456     104   ip_output+0x63/0x110
>  18)     3352      72   __ip_queue_xmit+0x16f/0x470
>  19)     3280     232   __tcp_transmit_skb+0xcb5/0x1140
>  20)     3048     120   tcp_write_xmit+0x5aa/0x1760
>  21)     2928      24   __tcp_push_pending_frames+0x39/0x110
>  22)     2904     104   tcp_rcv_established+0x37f/0xeb0
>  23)     2800      32   tcp_v4_do_rcv+0x13f/0x300
>  24)     2768     168   tcp_v4_rcv+0xc01/0x1300
>  25)     2600      48   ip_protocol_deliver_rcu+0x35/0x1b0
>  26)     2552      24   ip_local_deliver_finish+0x85/0x100
>  27)     2528      40   __netif_receive_skb_one_core+0x85/0xa0
>  28)     2488      56   netif_receive_skb+0x127/0x180
>  29)     2432     104   br_handle_frame_finish+0x428/0x680 [bridge]
>  30)     2328      40   br_handle_frame+0x132/0x2a0 [bridge]
>  31)     2288     280   __netif_receive_skb_core.constprop.0+0x16e/0xdf0
>  32)     2008      40   __netif_receive_skb_one_core+0x39/0xa0
>  33)     1968      56   netif_receive_skb+0x127/0x180
>  34)     1912     256   tun_get_user+0xbd0/0x1260 [tun]
>  35)     1656      56   tun_chr_write_iter+0x77/0xba [tun]
>  36)     1600      88   do_iter_readv_writev+0x161/0x260
>  37)     1512     256   vfs_writev+0x168/0x3c0
>  38)     1256      80   do_writev+0x7f/0x110

>  39)     1176     984   do_syscall_64+0xaa/0x670

If one worries about stack size, they may want to turn off
CONFIG_RANDOMIZE_KSTACK_OFFSET as I see some big numbers for do_syscall_64()

-- Steve


>  40)      192     192   entry_SYSCALL_64_after_hwframe+0x76/0x7e