linux-kernel - Re: [PATCH] binder_alloc: Move alloc_page() out of mmap

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Message-Id: <20240903110109.1696-1-hdanton@sina.com>
Date: Tue,  3 Sep 2024 19:01:09 +0800
From: Hillf Danton <hdanton@...a.com>
To: Barry Song <21cnbao@...il.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	"Barry Song" <v-songbaohua@...o.com>,
	"Carlos Llamas" <cmllamas@...gle.com>,
	"Suren Baghdasaryan" <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	"Tangquan Zheng" <zhengtangquan@...o.com>
Subject: Re: [PATCH] binder_alloc: Move alloc_page() out of mmap_rwsem to reduce the lock duration

On Tue, Sep 03, 2024 at 10:50:09AM +1200, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> The mmap_write_lock() can block all access to the VMAs, for example page
> faults. Performing memory allocation while holding this lock may trigger
> direct reclamation, leading to others being queued in the rwsem for an
> extended period.
> We've observed that the allocation can sometimes take more than 300ms,
> significantly blocking other threads. The user interface sometimes
> becomes less responsive as a result. To prevent this, let's move the
> allocation outside of the write lock.

I suspect concurrent allocators make things better wrt response, cutting
alloc latency down to 10ms for instance in your scenario. Feel free to
show figures given Tangquan's 48-hour profiling.

> A potential side effect could be an extra alloc_page() for the second
> thread executing binder_install_single_page() while the first thread
> has done it earlier. However, according to Tangquan's 48-hour profiling
> using monkey, the likelihood of this occurring is minimal, with a ratio
> of only 1 in 2400. Compared to the significantly costly rwsem, this is
> negligible.
> On the other hand, holding a write lock without making any VMA
> modifications appears questionable and likely incorrect. While this
> patch focuses on reducing the lock duration, future updates may aim
> to eliminate the write lock entirely.

If spin, better not before taking a look at vm_insert_page().