linux-kernel - Re: [PATCH] uprobes: Improve the usage of xol slots for better scalability

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Message-ID: <e9a3066a-ff07-6d57-9b97-17ecebe95e59@huawei.com>
Date: Fri, 27 Sep 2024 18:01:39 +0800
From: "Liao, Chang" <liaochang1@...wei.com>
To: Andi Kleen <ak@...ux.intel.com>
CC: <mhiramat@...nel.org>, <oleg@...hat.com>, <andrii@...nel.org>,
	<peterz@...radead.org>, <mingo@...hat.com>, <acme@...nel.org>,
	<namhyung@...nel.org>, <mark.rutland@....com>,
	<alexander.shishkin@...ux.intel.com>, <jolsa@...nel.org>,
	<irogers@...gle.com>, <adrian.hunter@...el.com>, <kan.liang@...ux.intel.com>,
	<linux-kernel@...r.kernel.org>, <linux-trace-kernel@...r.kernel.org>,
	<linux-perf-users@...r.kernel.org>, <bpf@...r.kernel.org>
Subject: Re: [PATCH] uprobes: Improve the usage of xol slots for better
 scalability



在 2024/9/24 7:52, Andi Kleen 写道:
>> Thanks for the suggestions, I will experiment with a read-write lock, meanwhile,
>> adding the documentation and testing for the lockless scheme.
> 
> Read-write locks are usually not worth it for short critical sections,
> in fact they can be slower due to cache line effects.

OK, I will start from a simple spinlock.

> 
>> Sorry, I may not probably get the point clear here, and it would be very
>> nice if more details are provided for the concern. Do you mean it's necessary
>> to make the if-body excution exclusive among the CPUs? If that's the case,
>> I guess the test_and_put_task_slot() is the equvialent to the race condition
>> check. test_and_put_task_slot() uses a compare and exchange operation on the
>> slot_ref of utask instance. Regardless of the work type being performed by
>> other CPU, it will always bail out unless the slot_ref has a value of one,
>> indicating the utask is free to access from local CPU.
> 
> What I meant is that the typical pattern for handling races in destruction
> is to detect someone else is racing and then let it do the destruction
> work or reacquire the resource (so just bail out).

Agreed.

> 
> But that's not what you're doing here, in fact you're adding a
> completely new code path that has different semantics? I haven't checked
> all the code, but it looks dubious.

Andi, I've just sent v2. Looking forward to your feedback. Thanks.

  https://lore.kernel.org/all/20240927094549.3382916-1-liaochang1@huawei.com/

> 
> -Andi
> 

-- 
BR
Liao, Chang