Many time we have got requests from end users and site owners to change the out of box site logo's behavior so that when they select it, it should take them to the home page of the root site collection. And this is a very fair request and the common behavior of most of the websites present in the internet. The default behavior of the SharePoint site logo is to take you to home page of the current site from where you have selected the logo. To change it open the Master Page of your site in Edit mode in SharePoint Designer.
Search for the div id 's4-workspace'. Little inside that div you are going to find a Table Column with class 's4-titlelogo' and "SharePoint:SPLinkButton runat="server" NavigateUrl="~site/"... inside that column.
Just change the NavigateUrl property value to "~siteCollection/" and save and publish the master page.
Now go back to your site, navigate to any sub-site and from inside that sub-site, click on the site logo and you will notice it will take you to the home page of the root site collection. Thats it!