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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8BIT Hi Prateek, On Mon, 16 Feb 2026, K Prateek Nayak wrote: > Hello Shubhang, > > On 2/14/2026 12:50 PM, Shubhang Kaushik wrote: >> Hi Peter, >> >> On Fri, 30 Jan 2026, Peter Zijlstra wrote: >> >>> Two issues related to reweight_entity() were raised; poking at all that got me >>> these patches. >>> >>> They're in queue.git/sched/core and I spend most of yesterday staring at traces >>> trying to find anything wrong. So far, so good. >>> >>> Please test. >>> >>> >> >> I’m seeing a consistent NULL pointer dereference in pick_task_fair() when running hackbench on an Ampere Altra (80 cores arm64). This is happening after applying the complete patchset on the latest 6.19.0+ kernel with PREEMPT_DYNAMIC (full), CONFIG_SCHED_CLUSTER and NOHZ_FULL enabled. > > Can you confirm you using the latest changes from: > > git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git sched/core > > at commit bdba3187771c ("sched/fair: Use full weight to __calc_delta()") > since this series has undergone some churn throughout the week? > Okay. I was not using commit bdba3187771c in my first test. After updating to that commit on sched/core, the issue is resolved on my machine. >> >> The system triggers a level 2 translation fault because pick_eevdf() returns NULL despite the runqueue having active tasks (cfs_rq->nr_running >>> 0). When pick_next_task_fair() attempts to dereference this NULL pointer >> to access the task structure, the kernel Oopses at pick_task_fair+0x48/0x148. >> >> pick_task_fair <- pick_eevdf() <- [active tasks] >> >> The root cause is an underflow in reweight_entity(): >> se->vprot -= avruntime; > > But how does it cause failure to pick? All vrpot does at pick is: > > /* curr needs to be queued and eligible first. */ > if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr))) > curr = NULL; > Because of the precision drift, the min_vruntime jumps ahead of the actual task runtime. This made the task look ineligible, so curr was being set to NULL. > /* vprot only helps for early return below. */ > if (curr && protect && protect_slice(curr)) > return curr; > Since curr was already NULL from the step above, the code skipped this protection check entirely. It then tried to search the rest of the tree for a new task. But again, as the math was broken it found nothing and returned NULL. That’s why the kernel crashed. > > Worst case "curr" runs till it loses eligibility even if vprot wraps > and becomes a large value and moreover, wouldn't vruntime_cmp() in > protect_slice() which compares the signed difference catch it? > > -- > Thanks and Regards, > Prateek > > The vruntime_cmp() should the handle the math, but the drift made the comparison logic invalid. The rounding errors in set_protect_slice() caused vprot and vruntime to be out of sync, breaking the eligibility invariants. Commit bdba3187771c restores the symmetry using 64-bit math which prevents the NULL return. Regards, Shubhang Kaushik --1275739760-600965611-1771365736=:14593--