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Date:   Tue, 26 Mar 2019 15:56:26 +0800
From:   Aaron Lu <aaron.lu@...ux.alibaba.com>
To:     Subhra Mazumdar <subhra.mazumdar@...cle.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Turner <pjt@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Fr?d?ric Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>, kerrnel@...gle.com
Subject: Re: [RFC][PATCH 00/16] sched: Core scheduling

On Tue, Mar 26, 2019 at 03:32:12PM +0800, Aaron Lu wrote:
> On Fri, Mar 08, 2019 at 11:44:01AM -0800, Subhra Mazumdar wrote:
> > 
> > On 2/22/19 4:45 AM, Mel Gorman wrote:
> > >On Mon, Feb 18, 2019 at 09:49:10AM -0800, Linus Torvalds wrote:
> > >>On Mon, Feb 18, 2019 at 9:40 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > >>>However; whichever way around you turn this cookie; it is expensive and nasty.
> > >>Do you (or anybody else) have numbers for real loads?
> > >>
> > >>Because performance is all that matters. If performance is bad, then
> > >>it's pointless, since just turning off SMT is the answer.
> > >>
> > >I tried to do a comparison between tip/master, ht disabled and this series
> > >putting test workloads into a tagged cgroup but unfortunately it failed
> > >
> > >[  156.978682] BUG: unable to handle kernel NULL pointer dereference at 0000000000000058
> > >[  156.986597] #PF error: [normal kernel read fault]
> > >[  156.991343] PGD 0 P4D 0
> > >[  156.993905] Oops: 0000 [#1] SMP PTI
> > >[  156.997438] CPU: 15 PID: 0 Comm: swapper/15 Not tainted 5.0.0-rc7-schedcore-v1r1 #1
> > >[  157.005161] Hardware name: SGI.COM C2112-4GP3/X10DRT-P-Series, BIOS 2.0a 05/09/2016
> > >[  157.012896] RIP: 0010:wakeup_preempt_entity.isra.70+0x9/0x50
> > >[  157.018613] Code: 00 be c0 82 60 00 e9 86 02 1a 00 66 0f 1f 44 00 00 48 c1 e7 03 be c0 80 60 00 e9 72 02 1a 00 66 90 0f 1f 44 00 00
> > >  53 48 89 fb <48> 2b 5e 58 48 85 db 7e 2c 48 81 3e 00 00 10 00 8b 05 a9 b7 19 01
> > >[  157.037544] RSP: 0018:ffffc9000c5bbde8 EFLAGS: 00010086
> > >[  157.042819] RAX: ffff88810f5f6a00 RBX: 00000001547f175c RCX: 0000000000000001
> > >[  157.050015] RDX: ffff88bf3bdb0a40 RSI: 0000000000000000 RDI: 00000001547f175c
> > >[  157.057215] RBP: ffff88bf7fae32c0 R08: 000000000001e358 R09: ffff88810fb9f000
> > >[  157.064410] R10: ffffc9000c5bbe08 R11: ffff88810fb9f5c4 R12: 0000000000000000
> > >[  157.071611] R13: ffff88bf4e3ea0c0 R14: 0000000000000000 R15: ffff88bf4e3ea7a8
> > >[  157.078814] FS:  0000000000000000(0000) GS:ffff88bf7f5c0000(0000) knlGS:0000000000000000
> > >[  157.086977] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > >[  157.092779] CR2: 0000000000000058 CR3: 000000000220e005 CR4: 00000000003606e0
> > >[  157.099979] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > >[  157.109529] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > >[  157.119058] Call Trace:
> > >[  157.123865]  pick_next_entity+0x61/0x110
> > >[  157.130137]  pick_task_fair+0x4b/0x90
> > >[  157.136124]  __schedule+0x365/0x12c0
> > >[  157.141985]  schedule_idle+0x1e/0x40
> > >[  157.147822]  do_idle+0x166/0x280
> > >[  157.153275]  cpu_startup_entry+0x19/0x20
> > >[  157.159420]  start_secondary+0x17a/0x1d0
> > >[  157.165568]  secondary_startup_64+0xa4/0xb0
> > >[  157.171985] Modules linked in: af_packet iscsi_ibft iscsi_boot_sysfs msr intel_rapl sb_edac x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel kvm ipmi_ssif irqbypass crc32_pclmul ghash_clmulni_intel ixgbe aesni_intel xfrm_algo iTCO_wdt joydev iTCO_vendor_support libphy igb aes_x86_64 crypto_simd ptp cryptd mei_me mdio pps_core ioatdma glue_helper pcspkr ipmi_si lpc_ich i2c_i801 mei dca ipmi_devintf ipmi_msghandler acpi_pad pcc_cpufreq button btrfs libcrc32c xor zstd_decompress zstd_compress raid6_pq hid_generic usbhid ast i2c_algo_bit drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops xhci_pci crc32c_intel ehci_pci ttm xhci_hcd ehci_hcd drm ahci usbcore mpt3sas libahci raid_class scsi_transport_sas wmi sg nbd dm_multipath dm_mod scsi_dh_rdac scsi_dh_emc scsi_dh_alua
> > >[  157.258990] CR2: 0000000000000058
> > >[  157.264961] ---[ end trace a301ac5e3ee86fde ]---
> > >[  157.283719] RIP: 0010:wakeup_preempt_entity.isra.70+0x9/0x50
> > >[  157.291967] Code: 00 be c0 82 60 00 e9 86 02 1a 00 66 0f 1f 44 00 00 48 c1 e7 03 be c0 80 60 00 e9 72 02 1a 00 66 90 0f 1f 44 00 00 53 48 89 fb <48> 2b 5e 58 48 85 db 7e 2c 48 81 3e 00 00 10 00 8b 05 a9 b7 19 01
> > >[  157.316121] RSP: 0018:ffffc9000c5bbde8 EFLAGS: 00010086
> > >[  157.324060] RAX: ffff88810f5f6a00 RBX: 00000001547f175c RCX: 0000000000000001
> > >[  157.333932] RDX: ffff88bf3bdb0a40 RSI: 0000000000000000 RDI: 00000001547f175c
> > >[  157.343795] RBP: ffff88bf7fae32c0 R08: 000000000001e358 R09: ffff88810fb9f000
> > >[  157.353634] R10: ffffc9000c5bbe08 R11: ffff88810fb9f5c4 R12: 0000000000000000
> > >[  157.363506] R13: ffff88bf4e3ea0c0 R14: 0000000000000000 R15: ffff88bf4e3ea7a8
> > >[  157.373395] FS:  0000000000000000(0000) GS:ffff88bf7f5c0000(0000) knlGS:0000000000000000
> > >[  157.384238] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > >[  157.392709] CR2: 0000000000000058 CR3: 000000000220e005 CR4: 00000000003606e0
> > >[  157.402601] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > >[  157.412488] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > >[  157.422334] Kernel panic - not syncing: Attempted to kill the idle task!
> > >[  158.529804] Shutting down cpus with NMI
> > >[  158.573249] Kernel Offset: disabled
> > >[  158.586198] ---[ end Kernel panic - not syncing: Attempted to kill the idle task! ]---
> > >
> > >RIP translates to kernel/sched/fair.c:6819
> > >
> > >static int
> > >wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
> > >{
> > >         s64 gran, vdiff = curr->vruntime - se->vruntime; /* LINE 6819 */
> > >
> > >         if (vdiff <= 0)
> > >                 return -1;
> > >
> > >         gran = wakeup_gran(se);
> > >         if (vdiff > gran)
> > >                 return 1;
> > >}
> > >
> > >I haven't tried debugging it yet.
> > >
> > I think the following fix, while trivial, is the right fix for the NULL
> > dereference in this case. This bug is reproducible with patch 14. I
> 
> I assume you meant patch 4?

Correction, should be patch 9 where pick_task_fair() is introduced.

Thanks,
Aaron

> 
> My understanding is, this is due to 'left' being NULL in
> pick_next_entity().
> 
> With patch 4, in pick_task_fair(), pick_next_entity() can be called with
> an empty rbtree of cfs_rq and with a NULL 'curr'. This resulted in a
> NULL 'left'. Before patch 4, this can't happen.
> 
> It's not clear to me why NULL is used instead of 'curr' for
> pick_next_entity() in pick_task_fair(). My first thought is, 'curr' will
> not be considered as next entity, but then 'curr' is checked after
> pick_next_entity() returns so this shouldn't be the reason. Guess I
> missed something.
> 
> Thanks,
> Aaron
> 
> > also did
> > some performance bisecting and with patch 14 performance is
> > decimated, that's
> > expected. Most of the performance recovery happens in patch 15 which,
> > unfortunately, is also the one that introduces the hard lockup.
> > 
> > -------8<-----------
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 1d0dac4..ecadf36 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4131,7 +4131,7 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct
> > sched_entity *curr)
> >          * Avoid running the skip buddy, if running something else can
> >          * be done without getting too unfair.
> > */
> > -       if (cfs_rq->skip == se) {
> > +       if (cfs_rq->skip && cfs_rq->skip == se) {
> >                 struct sched_entity *second;
> > 
> >                 if (se == curr) {
> > @@ -4149,13 +4149,15 @@ pick_next_entity(struct cfs_rq *cfs_rq,
> > struct sched_entity *curr)
> > /*
> >          * Prefer last buddy, try to return the CPU to a preempted task.
> > */
> > -       if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
> > +       if (left && cfs_rq->last &&
> > wakeup_preempt_entity(cfs_rq->last, left)
> > +           < 1)
> >                 se = cfs_rq->last;
> > 
> > /*
> >          * Someone really wants this to run. If it's not unfair, run it.
> > */
> > -       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
> > +       if (left && cfs_rq->next &&
> > wakeup_preempt_entity(cfs_rq->next, left)
> > +           < 1)
> >                 se = cfs_rq->next;
> > 
> >         clear_buddies(cfs_rq, se);
> > 

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