Hello,
(Cc Daniel)
On (07/06/17 19:28), Tetsuo Handa wrote:
> (...snipped...)
> [ 912.892027] kworker/3:3 D12120 3217 2 0x00000080
> [ 912.892041] Workqueue: events console_callback
> [ 912.892042] Call Trace:
> [ 912.892047] __schedule+0x23f/0x5d0
> [ 912.892051] schedule+0x31/0x80
> [ 912.892056] schedule_preempt_disabled+0x9/0x10
> [ 912.892061] __mutex_lock.isra.2+0x2ac/0x4d0
> [ 912.892068] __mutex_lock_slowpath+0xe/0x10
> [ 912.892072] ? __mutex_lock_slowpath+0xe/0x10
> [ 912.892077] mutex_lock+0x2a/0x30
> [ 912.892105] vmw_fb_pan_display+0x35/0x90 [vmwgfx]
> [ 912.892114] fb_pan_display+0xca/0x160
> [ 912.892118] bit_update_start+0x1b/0x40
> [ 912.892123] fbcon_switch+0x4a6/0x630
> [ 912.892128] redraw_screen+0x15a/0x240
> [ 912.892132] ? update_attr.isra.3+0x90/0x90
> [ 912.892139] complete_change_console+0x3d/0xd0
> [ 912.892143] change_console+0x57/0x90
> [ 912.892147] console_callback+0x116/0x190
> [ 912.892153] process_one_work+0x1f5/0x390
> [ 912.892156] worker_thread+0x46/0x410
> [ 912.892161] ? __schedule+0x247/0x5d0
> [ 912.892165] kthread+0xff/0x140
> [ 912.892170] ? process_one_work+0x390/0x390
> [ 912.892174] ? kthread_create_on_node+0x60/0x60
> [ 912.892178] ? do_syscall_64+0x13a/0x140
> [ 912.892181] ret_from_fork+0x25/0x30
> (...snipped...)
> [ 912.934633] kworker/0:0 D12824 4263 2 0x00000080
> [ 912.934643] Workqueue: events vmw_fb_dirty_flush [vmwgfx]
> [ 912.934643] Call Trace:
> [ 912.934645] __schedule+0x23f/0x5d0
> [ 912.934646] schedule+0x31/0x80
> [ 912.934647] schedule_timeout+0x189/0x290
> [ 912.934649] ? del_timer_sync+0x40/0x40
> [ 912.934650] io_schedule_timeout+0x19/0x40
> [ 912.934651] ? io_schedule_timeout+0x19/0x40
> [ 912.934653] congestion_wait+0x7d/0xd0
> [ 912.934654] ? wait_woken+0x80/0x80
> [ 912.934654] shrink_inactive_list+0x3e3/0x4d0
> [ 912.934656] shrink_node_memcg+0x360/0x780
> [ 912.934657] ? list_lru_count_one+0x65/0x70
> [ 912.934658] shrink_node+0xdc/0x310
> [ 912.934658] ? shrink_node+0xdc/0x310
> [ 912.934659] do_try_to_free_pages+0xea/0x370
> [ 912.934660] try_to_free_pages+0xc3/0x100
> [ 912.934661] __alloc_pages_slowpath+0x441/0xd50
> [ 912.934663] ? ___slab_alloc+0x1b6/0x590
> [ 912.934664] __alloc_pages_nodemask+0x20c/0x250
> [ 912.934665] alloc_pages_current+0x65/0xd0
> [ 912.934666] new_slab+0x472/0x600
> [ 912.934668] ___slab_alloc+0x41b/0x590
> [ 912.934685] ? drm_modeset_lock_all+0x1b/0xa0 [drm]
> [ 912.934691] ? drm_modeset_lock_all+0x1b/0xa0 [drm]
> [ 912.934692] __slab_alloc+0x1b/0x30
> [ 912.934693] ? __slab_alloc+0x1b/0x30
> [ 912.934694] kmem_cache_alloc+0x16b/0x1c0
> [ 912.934699] drm_modeset_lock_all+0x1b/0xa0 [drm]
> [ 912.934702] vmw_framebuffer_dmabuf_dirty+0x47/0x1d0 [vmwgfx]
> [ 912.934706] vmw_fb_dirty_flush+0x229/0x270 [vmwgfx]
> [ 912.934708] process_one_work+0x1f5/0x390
> [ 912.934709] worker_thread+0x46/0x410
> [ 912.934710] ? __schedule+0x247/0x5d0
> [ 912.934711] kthread+0xff/0x140
> [ 912.934712] ? process_one_work+0x390/0x390
> [ 912.934713] ? kthread_create_on_node+0x60/0x60
> [ 912.934714] ret_from_fork+0x25/0x30
>
> Pressing SysRq-c caused all locks to be released (doesn't it ?), and console
hm, I think what happened is a bit different thing. sysrq-c didn't
unlock any of the locks. I suspect that ->bo_mutex is never taken
on the direct path vprintk_emit()->console_unlock()->call_console_drivers(),
otherwise it would have made vprintk_emit() from atomic context impossible.
so ->bo_mutex does not directly affect printk. it affects it indirectly.
the root cause, however, I think, is actually console semaphore and
console_lock() in change_console(). printk() depends on it a lot, so do
drm/tty/etc. as long as the console semaphore is locked, printk can only
add new messages to the logbuf. and this is what happened here, under
console_sem we scheduled on ->bo_mutex, which was locked because of memory
allocation on another CPU, yes. you see lost messages in your report
because part of printk that is responsible for storing new messages was
working just fine; it's the output to consoles that was blocked by
console_sem -> bo_mutex chain.
the reason why sysrq-c helped was because, sysrq-c did
panic_on_oops = 1
panic()
and panic() called console_flush_on_panic(), which completely ignored the
state of console semaphore and just flushed all the pending logbuf
messages.
console_trylock();
console_unlock();
so, I believe, console_semaphore remained locked just like it was before
sysrq-c, and ->bo_mutex most likely remained locked as well. it's just we
ignored the state of console_sem and this let us to print the messages
(which also proves that ->bo_mutex is not taken by
console_unlock()->call_console_drivers()).
[..]
> Since vmw_fb_dirty_flush was stalling for 130989 jiffies,
> vmw_fb_dirty_flush started stalling at uptime = 782. And
> drm_modeset_lock_all() from vmw_fb_dirty_flush work started
> GFP_KERNEL memory allocation
>
> ----------
> void drm_modeset_lock_all(struct drm_device *dev)
> {
> struct drm_mode_config *config = &dev->mode_config;
> struct drm_modeset_acquire_ctx *ctx;
> int ret;
>
> ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
> if (WARN_ON(!ctx))
> return;
hm, this allocation, per se, looks ok to me. can't really blame it.
what you had is a combination of factors
CPU0 CPU1 CPU2
console_callback()
console_lock()
^^^^^^^^^^^^^
vprintk_emit() mutex_lock(&par->bo_mutex)
kzalloc(GFP_KERNEL)
console_trylock() kmem_cache_alloc() mutex_lock(&par->bo_mutex)
^^^^^^^^^^^^^^^^ io_schedule_timeout
// but I haven't seen the logs that you have provided, yet.
[..]
> As a result, console was not able to print SysRq-t output.
>
> So, how should we avoid this problem?
from the top of my head -- console_sem must be replaced with something
better. but that's a task for years.
hm...
> But should fbcon, drm, tty and so on stop using __GFP_DIRECT_RECLAIM
> memory allocations because consoles should be as responsive as printk() ?
may be, may be not. like I said, the allocation in question does not
participate in console output. it's rather hard to imagine how we would
enforce a !__GFP_DIRECT_RECLAIM requirement here. it's console semaphore
to blame, I think.
if we could unlock console for some of operations done under ->bo_mutex,
then may be we could make some printing progress, at least. but I have
zero knowledge of that part of the kernel.
-ss
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