Re: Re: [PATCH V2] rcu: Make sure new krcp free business is handled after the wanted rcu grace period.

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Apr 04, 2023 at 09:33:07PM +0200, Uladzislau Rezki wrote:
> > > > My concern is that running the channels separately might mean more grace
> > > > periods (and thus more energy draw) on nearly idle devices, such devices
> > > > usually being the ones for which energy efficiency matters most.
> > > > 
> > > > But perhaps Vlad, Neeraj, or Joel has some insight on this, given
> > > > that they are the ones working on battery-powered devices.
> > > > 
> > > > > > Either way, this fixes only one bug of two.  The second bug is in the
> > > > > > kfree_rcu() tests, which should have caught this bug.  Thoughts on a good fix
> > > > > > for those tests?
> > > > >
> > > > > I inserted a msleep() between "rcu_read_lock(), get pointer via rcu_dereference()"
> > > > > and "reference pointer, using the member", at the rcu scenario, then we can
> > > > > reproduce this issue very soon in stress test. Can kfree_rcu() tests insert msleep()?
> > > > 
> > > > Another approach is to separate concerns, so that readers interact with
> > > > grace periods in the rcutorture.c tests, and to add the interaction
> > > > of to-be-freed memory with grace periods in the rcuscale kvfree tests.
> > > > I took a step in this direction with this commit on the -rcu tree's
> > > > "dev" branch:
> > > > 
> > > > efbe7927f479 ("rcu/kvfree: Add debug to check grace periods")
> > > > 
> > > > Given this, might it be possible to make rcuscale.c's kfree_rcu()
> > > > testing create patterns of usage of the three channels so as to
> > > > catch this bug that way?
> > > > 
> > > 
> > > I can try it on my k5.15 device, and need some time.
> > > I have a question. Do you mean add code in tree.c to create pattern
> > > while channel data is being freed?
> > > If so, both rcuscales.c and tree.c need to be modified for the test case.
> > 
> > My thought is to run the test on a system where very little else is
> > happening, and then creating the temporal pattern only in rcuscale.c.
> > One way would be to modify kfree_scale_thread(), perhaps using an
> > additional module parameter using torture_param().
> > 
> > But just out of curiosity, what changes were you thinking of making
> > in tree.c?
> > 
> OK. I can reproduce it on latest rcu-dev:
> 
> <snip>
> [   75.302795] ------------[ cut here ]------------
> [   75.302801] WARNING: CPU: 50 PID: 721 at kernel/rcu/tree.c:3043 kfree_rcu_work+0x157/0x1a0
> [   75.302808] Modules linked in: test_vmalloc(E+) bochs(E) drm_vram_helper(E) snd_pcm(E) drm_ttm_helper(E) ppdev(E) snd_timer(E) joydev(E) ttm(E) drm_kms_helper(E) snd(E) parport_pc(E) soundcore(E) evdev(E) pcspkr(E) sg(E) serio_raw(E) parport(E) drm(E) qemu_fw_cfg(E) button(E) ip_tables(E) x_tables(E) autofs4(E) ext4(E) crc32c_generic(E) crc16(E) mbcache(E) jbd2(E) sd_mod(E) t10_pi(E) crc64_rocksoft(E) crc64(E) crc_t10dif(E) crct10dif_generic(E) sr_mod(E) cdrom(E) crct10dif_common(E) ata_generic(E) ata_piix(E) libata(E) scsi_mod(E) psmouse(E) e1000(E) scsi_common(E) i2c_piix4(E) floppy(E)
> [   75.302865] CPU: 50 PID: 721 Comm: kworker/50:1 Kdump: loaded Tainted: G            E      6.3.0-rc1+ #58
> [   75.302868] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
> [   75.302870] Workqueue: events kfree_rcu_work
> [   75.302905] RIP: 0010:kfree_rcu_work+0x157/0x1a0
> [   75.302907] Code: 8b 05 75 f9 37 01 4c 29 e8 48 83 f8 f8 76 40 48 8b 4c 24 08 48 83 f9 01 74 35 48 8b 05 ca b4 44 01 48 29 c8 48 83 f8 f8 76 25 <0f> 0b 48 8b 44 24 38 65 48 2b 04 25 28 00 00 00 75 23 48 83 c4 40
> [   75.302910] RSP: 0018:ffffbd4642d8bde8 EFLAGS: 00010202
> [   75.302913] RAX: fffffffffffffffc RBX: ffff9f693d5dd140 RCX: 000000000000003c
> [   75.302914] RDX: 0000000000000002 RSI: ffffbd4642d8be08 RDI: ffff9f5a4d608000
> [   75.302916] RBP: ffffbd4642d8be08 R08: 0000001188654ff5 R09: 0000000000000000
> [   75.302918] R10: 0000000000000001 R11: 0000000000000001 R12: ffffbd46812d7000
> [   75.302919] R13: 0000000000000260 R14: ffffbd4642d8bdf8 R15: ffff9f5a47637000
> [   75.302922] FS:  0000000000000000(0000) GS:ffff9f693e200000(0000) knlGS:0000000000000000
> [   75.302924] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   75.302926] CR2: 0000562dfe4307d0 CR3: 000000054ba26000 CR4: 00000000000006e0
> [   75.302930] Call Trace:
> [   75.302937]  <TASK>
> [   75.302942]  ? lock_acquire+0xc8/0x1a0
> [   75.302949]  process_one_work+0x29d/0x560
> [   75.302957]  ? __pfx_worker_thread+0x10/0x10
> [   75.302960]  worker_thread+0x52/0x3a0
> [   75.302964]  ? __pfx_worker_thread+0x10/0x10
> [   75.302967]  kthread+0xe7/0x110
> [   75.302970]  ? __pfx_kthread+0x10/0x10
> [   75.302973]  ret_from_fork+0x2c/0x50
> [   75.302984]  </TASK>
> [   75.302986] ---[ end trace 0000000000000000 ]---
> <snip>
> 
> This is with:
> 
> <snip>
> commit 8f6414680a0d539ca0e7fde80556c71b7b3da88a (HEAD -> dev)
> Author: Uladzislau Rezki (Sony) <urezki@gmail.com>
> Date:   Tue Apr 4 15:51:56 2023 +0200
> 
>     rcu/kvfree: Add debug check of GP ready for ptrs in a list
> 
> commit efbe7927f47958a6805da5560d9a5f469ba51e73 (origin/dev)
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date:   Mon Apr 3 16:49:14 2023 -0700
> 
>     rcu/kvfree: Add debug to check grace periods
> 
> + below revert
> 
> commit 6b4fef6ec689b1dda9c63be77e9a81a52cc39dc1
> Author: Ziwei Dai <ziwei.dai@unisoc.com>
> Date:   Fri Mar 31 20:42:09 2023 +0800
> 
>     rcu/kvfree: Avoid freeing new kfree_rcu() memory after old grace period
> <snip>
> 
> The test is "sudo ./test_vmalloc.sh run_test_mask=768 nr_threads=64&"
> 
> it runs single argument and double argument to free vmalloc ptrs.,
> number of threads are 64:
> 
> without revert(with a patch that is in question), i am not able to
> reproduce it anymore.

Very good!!!

This test does not fit very will into the rcutorture script framework,
but might it be able to guide changes to rcuscale.c?

							Thanx, Paul