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

[Steven Rostedt]

On Fri, 28 Aug 2026 15:13:13 -0400
Steven Rostedt <rostedt at goodmis.org> wrote:

> 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()

And the scheduler has some heavy stack usage here!

  # trace-cmd stack
(stack tracer running)
        Depth    Size   Location    (31 entries)
        -----    ----   --------
  0)     8120      48   __msecs_to_jiffies+0x9/0x30
  1)     8072     104   update_group_capacity+0x94/0x960
  2)     7968     528   update_sd_lb_stats.constprop.0+0x426/0x39b0
  3)     7440     424   sched_balance_find_src_group+0x8f/0x1150
  4)     7016     552   sched_balance_rq+0x934/0x4130
  5)     6464     408   pick_task_fair+0xa71/0x1fe0
  6)     6056     496   __schedule+0x628/0x76f0
  7)     5560      32   schedule+0xde/0x2c0
  8)     5528      32   io_schedule+0x8c/0x100
  9)     5496     256   rq_qos_wait+0x12b/0x230
 10)     5240     136   wbt_wait+0x150/0x260
 11)     5104      40   __rq_qos_throttle+0x51/0xa0
 12)     5064     304   blk_mq_submit_bio+0xc40/0x28e0
 13)     4760     240   submit_bio_noacct_nocheck+0x410/0xb30
 14)     4520      40   ext4_io_submit+0xca/0x1a0
 15)     4480     144   ext4_bio_write_folio+0x5e2/0x1470
 16)     4336      96   mpage_process_page_bufs+0x392/0x700
 17)     4240     616   mpage_prepare_extent_to_map+0xaae/0x1120
 18)     3624     480   ext4_do_writepages+0x971/0x37f0
 19)     3144     312   ext4_writepages+0x2d2/0x610
 20)     2832     152   do_writepages+0x21e/0x560
 21)     2680     184   __writeback_single_inode+0x119/0x1240
 22)     2496     752   writeback_sb_inodes+0x67d/0x17b0
 23)     1744     168   __writeback_inodes_wb+0xf2/0x270
 24)     1576     304   wb_writeback+0x63e/0x890
 25)     1272     320   wb_workfn+0x844/0xca0
 26)      952     360   process_one_work+0x8b4/0x15d0
 27)      592     176   worker_thread+0x5d3/0xfb0
 28)      416      64   kthread+0x33c/0x420
 29)      352     160   ret_from_fork+0x65c/0x9e0
 30)      192     192   ret_from_fork_asm+0x1a/0x30

Note, I'm running this on my server. That is, this is a production machine.

-- Steve