Re: [PATCH] sched/deadline: Fix DL server divide-by-zero for inactive CPUs

Hello,

On 12/08/26 20:32, Hui Su wrote:
> Commit 4043f5498416 ("sched/deadline: Reject debugfs dl_server writes
> for offline CPUs") rejects per-CPU DL server parameter updates once the
> target CPU is offline. However, during CPU hot-unplug, the CPU is cleared
> from cpu_active_mask before it is marked offline.
> 
> This leaves a window where cpu_online() is still true while
> cpu_active() is already false. A debugfs write during this window passes
> the cpu_online() check in sched_server_write_common() and reaches
> dl_server_apply_params() with init=false.
> 
> dl_bw_cpus() counts the active CPUs in the root domain. For an isolated
> CPU whose root-domain span contains only that CPU, it returns zero once
> the CPU becomes inactive. If the server bandwidth is attached,
> dl_server_apply_params() then passes this zero CPU count to __dl_sub()
> and __dl_add(), both of which divide by the CPU count.
> 
> Using CPU1 with isolcpus=domain,1 and a temporary local hotplug pause
> hook to stop the teardown after cpu_active_mask was cleared but before
> the CPU became offline reproduced the state as:
> 
> dl_bw_cpus=0 attached=1 dl_b->bw=-1 total_bw=52428 span=1 active=0
> 
> Writing a new fair-server runtime while CPU1 was held in that state
> triggered:
> 
> # echo 40000000 > /sys/kernel/debug/sched/fair_server/cpu1/runtime
> 
> Oops: divide error: 0000 [#1] SMP NOPTI
> RIP: 0010:dl_server_apply_params+0x39d/0x400
> Call Trace:
> sched_server_write_common.isra.0+0x1d2/0x2d0
> full_proxy_write+0x64/0x90
> vfs_write+0xf7/0x540
> ksys_write+0x6e/0xf0
> 
> Reject DL server parameter writes when the target CPU is inactive, not
> only when it is offline.
> 
> Also update root-domain bandwidth in dl_server_apply_params() only while
> the target CPU is active. This second check is necessary because CPU
> hot-unplug can race with the debugfs path after its CPU state check and
> before dl_server_apply_params() updates the bandwidth.
> 
> Keep the runqueue-local utilization update independent of cpu_active()
> so that the local bandwidth state remains consistent if the CPU becomes
> inactive during the parameter update.
> 
> With the fix, a write during the same hot-unplug window is rejected with
> -EBUSY instead of reaching __dl_sub() or __dl_add() with a zero CPU
> count.
> 
> Fixes: d741f297bcea ("sched/fair: Fair server interface")
> Reported-by: Sashiko <[email protected]>
> Link: https://lore.kernel.org/r/[email protected]
> Cc: [email protected]
> Signed-off-by: Hui Su <[email protected]>
> ---

Looks good to me, thanks!

Acked-by: Juri Lelli <[email protected]>

Best,
Juri