linux-kernel - Re: [PATCH] blk-throttle: check policy bit in blk_throtl

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Message-ID: <tencent_9DE422078550681A63BE8AC4C6DE7CB29809@qq.com>
Date: Wed,  3 Sep 2025 10:55:55 +0800
From: Han Guangjiang <gj.han@...mail.com>
To: hailan@...uai.org.cn
Cc: axboe@...nel.dk,
	fanggeng@...iang.com,
	gj.han@...mail.com,
	hanguangjiang@...iang.com,
	liangjie@...iang.com,
	linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	yangchen11@...iang.com
Subject: Re: [PATCH] blk-throttle: check policy bit in blk_throtl_activated()

Hi Kuai,

> Instead of add checking from hot path, do you consider delaying setting q->td
> until policy is activated from the slow path? I think this is better solution.

Thank you for your review. You're absolutely right that performance 
considerations in the hot path are important.

We actually considered delaying the setting of q->td until after policy 
activation, but we found that q->td is needed by blkcg_activate_policy() 
during its execution, so it has to be set before calling 
blkcg_activate_policy().

We explored several alternative approaches:

1) Adding a dedicated flag like 'throttle_ready' to struct request_queue: 
   - Set this flag at the end of blk_throtl_init()
   - Check this flag in blk_throtl_activated() to determine if policy 
     loading is complete
   - However, this requires adding a new bool variable to the struct

2) Reusing the q->td pointer with low-order bit flags:
   - Use pointer low-order bits to mark initialization completion status
   - This would avoid adding new fields but requires careful handling 
     and additional processing

Given these constraints, we chose the current approach of checking the 
policy bit in blk_throtl_activated() as it:
- Doesn't require struct changes
- Provides a clean, atomic check
- Aligns with the existing policy activation mechanism

We would appreciate your suggestions on how to better handle this 
initialization race condition.

Thanks,
Han Guangjiang