linux-kernel - Re: [PATCH V7 4/4] softirq: Allow early break the softirq processing loop

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Date:   Sat, 26 Sep 2020 00:42:25 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     qianjun.kernel@...il.com, peterz@...radead.org, will@...nel.org,
        luto@...nel.org, linux-kernel@...r.kernel.org,
        laoar.shao@...il.com, qais.yousef@....com, urezki@...il.com
Subject: Re: [PATCH V7 4/4] softirq: Allow early break the softirq processing loop

On Fri, Sep 25 2020 at 02:42, Frederic Weisbecker wrote:

> On Thu, Sep 24, 2020 at 05:37:42PM +0200, Thomas Gleixner wrote:
>> Subject: softirq; Prevent starvation of higher softirq vectors
> [...]
>> +	/*
>> +	 * Word swap pending to move the not yet handled bits of the previous
>> +	 * run first and then clear the duplicates in the newly raised ones.
>> +	 */
>> +	swahw32s(&cur_pending);
>> +	pending = cur_pending & ~(cur_pending << SIRQ_PREV_SHIFT);
>> +
>>  	for_each_set_bit(vec_nr, &pending, NR_SOFTIRQS) {
>>  		int prev_count;
>>  
>> +		vec_nr &= SIRQ_VECTOR_MASK;
>
> Shouldn't NR_SOFTIRQS above protect from that?

It does, but that's wrong. The bitmap size in that for_each() loop must
obviously be SIRQ_PREV_SHIFT + NR_SOFTIRQS for this to work.

>> +	} else {
>> +		/*
>> +		 * Retain the unprocessed bits and swap @cur_pending back
>> +		 * into normal ordering
>> +		 */
>> +		cur_pending = (u32)pending;
>> +		swahw32s(&cur_pending);
>> +		/*
>> +		 * If the previous bits are done move the low word of
>> +		 * @pending into the high word so it's processed first.
>> +		 */
>> +		if (!(cur_pending & SIRQ_PREV_MASK))
>> +			cur_pending <<= SIRQ_PREV_SHIFT;
>
> If the previous bits are done and there is no timeout, should
> we consider to restart a loop?

We only enter this code path if there was a timeout. Otherwise pending
would be 0.

Thanks,

        tglx