Re: [PATCH v25 1/9] sched: Make class_schedulers avoid pushing current, and get rid of proxy_tag_curr()

[K Prateek Nayak]

Hello Peter,

On 3/18/2026 7:22 PM, Peter Zijlstra wrote:
>> @@ -7114,6 +7119,24 @@ static void __sched notrace __schedule(i
>>  				goto keep_resched;
>>  			}
>>  		}
>> +
>> +		/*
>> +		 * When transitioning like:
>> +		 *
>> +		 *	   prev		next
>> +		 * donor:    B		  B
>> +		 * curr:     A		  B

Even for curr going A -> C, we should do this.

Consider B is blocked on C which is in turn blocked on A. B gets picked
and proxies A first as a result of the wait chain (B -> C -> A)

A runs and does an unlock with a handoff to C clearing its blocked_on.
In schedule B is picked again but this time it proxies C.

A might be on the push list and was skipped last time around since
A->on_cpu = 1 but now that is gone and we should still do put_prev_task()
+ set_next_task().

>> +		 *
>> +		 * then put_prev_set_next_task() will not have done anything,
>> +		 * since B == B. However, A might have missed a RT/DL balance
>> +		 * opportunity due to being on_cpu.
>> +		 */
>> +		if (next == rq->donor && next == prev_donor) {
> 
> 	&& next != prev
> 
>> +			next->sched_class->put_prev_task(rq, next, next);
>> +			next->sched_class->set_next_task(rq, next, true);
>> +		}
>> +	} else {
>> +		rq_set_donor(rq, next);
>>  	}
>>  picked:
>>  	clear_tsk_need_resched(prev);

-- 
Thanks and Regards,
Prateek